Bruce Wayne/Batman for the most part wasn't a muscular type of a person that relied on strength. He wasn't built like a bodybuilder or weigh lifter. While strong and slim, his main attributes were an amazing agility, speed and reflexes. His athletic skills were more than exceptional, and he was even able to move and swing in the air like Spider-Man
Panel below from Batman #425
Panel below from Detective Comics #589
Panel below from Detective Comics #582
Panel below from Detective comics #614
Panel below from Detective Comics #444
He was even shown dodging bullets numerous times back in the 80's..
..and still is today
Bruce Wayne is officially 6'2 tall and weights 210 lbs according to the official Who's Who in DC Universe
The point is, it was Batman's agility, not the physical strength or muscles that were his prime attributes. In the very beginning though, it was somewhat different
The first time we get to see Batman shirtless is in 1939's Detective Comics #29 when he gets shot and goes to the doctor to fix the wound. He is shown as a man in shape but the early style of the comic books was very stylized and not as detailed. For example, the doctor is shown to be wide as well
It was Batman's origins, first appearing in 1939's Detective Comics #33, that flat out stated and showed that Bruce is incredibly strong and his body is a "physical perfection"
That physique was very consistent, even if the art was very stylized. Panel below from 1940's Batman #2
His exceptional strength became very apparent very quickly
Panel below from Detective Comics # 44
Panel below from Batman #3
The art got more and more detailed starting with late 60's, eventually morphing into the standard style for the comic books in the 70's. As the art changed and the Batman universe experienced its revolution, switching gears to more serious stories, Batman's physique wasn't that of a weightlifter anymore and his body was streamlined significantly. He was now shown as a man in a very good shape, but nothing exceptional or out of norm for a man his age and height. The change wasn't only visual - the stories for the first time humanized the character, showing him actually struggle with heavy things
Batman lifting someone in the 1940's...
and in the late 1960's
He was still well built and muscular, but he didnt have any exceptionally huge muscles and abs.
In other words his physique now was more human, more down to Earth, more common looking
As one can notice from those examples, Bruce is not a bodybuilder type, but is built like a typical man in shape. Strength and size weren't his calling cards, the swiftness in movement, fighting styles and acrobatic skills were.
This stature remained consistent for many years.
In the late 80's he was gaining more and more muscles
In 1991's Legends of the Dark Knight #16 Bruce was shown as being even more ripped, but the story still stressed out that he's no bodybuilder
Batman sometimes performed amazing fits and could have a boost of strength that would be almost impossible for a human being, but that was due to his strong and sudden rush of adrenaline and/or anger
Panels below from Detective Comics #614
Panel below from Shadow of the Bat #4
The mid/late 90's started changing Batman's body more and more. In time he was given much, much more strength and his body and muscles grew in size as the years progressed. Soon he was shown to perform such inhuman tasks as kicking in steel doors, ripping out prison bars and punching through the hood of the car. He was visibly bigger and could lift 1000 lbs as his daily routine
Panel below from Batman #655
Panel below from Batman Tenses
For a better comparison on how the comic books gradually toned down and then amped up Batman's physique over his long history, take a look at this comparison set
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THE 1966 SERIES
Adam West is the only actor to date to match Batman's 6'2 height in the comics. Also, while it was a bit hard to tell when in the costume, West was well built as well, even more so than his comic counterpart at the time. His physique actually matched that of the Bronze Age Batman, and he actually resembled him quite a bit
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THE TIM BURTON MOVIES
Michael Keaton is 5'10, about an average height for a Caucasian male and roughly four inches shorter than Batman’s height in the comics . Before playing Batman, Michael Keaton played a professional athlete in the 1986 movie called Touch and Go
The closest we get to see his physique around the Batman shoot is in the 1988 movie called Clean and Sober, in which we can see that Keaton was of a good shape of an average man. It's important to note that he went under extensive kick-boxing training for his role in Batman
Michael Keaton: So the preparation that I did, aside from basic things that I think you always have to do was getting in shape because I had to be very physical in this movie. Actually I couldn't get too big because I couldn't fit in the suit if I was too big and I couldn't get too small, I couldn't loose too much weight (MNC TV 1990)
In 1992 he appeared on SNL, seemingly bigger than in the late 80's
It's worth noting that Michael Keaton was able to achieve an impressive fit of gaining a great amount of muscle tissue for his role in the movie Desperate Measures
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JOEL SCHUMACHER MOVIES
Val Kilmer is 6 feet tall, two inches shorter than his comic book counterpart. Kilmer had a wide but athletic body, which very much resembles the Bronze/Modern Age I physique of Batman.
George Clooney is 5'11 tall. He was the skinniest and least physically fit of the actors who portrayed Bruce Wayne/Batman.
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CHRISTOPHER NOLAN MOVIES
Christian Bale always dives deep into his roles and commits to his characters both mentally and physically. In order to play Batman, he achieved a stunning fit of gaining incredible amount of weight and muscle tissue coming from a movie Machinist in which he was almost anorexic
Bale is 6'0 tall, two inches shorter than his comic book counterpart. He was shown as being more bulky, wider and more muscled than many of Batman's incarnations, mirroring more the recent depictions and the earliest years with his physique
Christopher Nolan: [Christian Bale] proceeded to put on a lot of weight in muscle terms, he really tried bulk up.(...) I sort of gently suggested to cut down a bit (Making of BB)
Christian Bale: I was just stuffing my face all day long, and lifting every weights and eating more and eventually went up to around 220 lbs. I just listened to what he said about get as big as you possibly can so thats what I did but I was kind of like a bear, I wasn't really like a martial arts guy, lean and ripped and everything like that. I could see the look on Chris' face, he looked at me and it was like 'oh' Christ, what has this guy done? (...) So I had to then loose a lot of weight (Making of BB)
Once they cast me, that’s when the real effort started just because that’s was when I actually had to start getting strong because he’s got no super powers. You have to look like you can be a brawler. He has no super powers and so he has to look like he really can do what you see him doing. And it was very… It was cutting it very fine, you know? I kind of just got ready in time for the beginning of the film.” (movies.about.com)
The weight fluctuation is somewhat noticeable in the movie as well
Bale was also the only non-American actor who portrayed Batman/Bruce Wayne in the theatrical movies
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