Comics, like motion pictures, believed in protecting their "stars" from injury through the use of doubles. The pay for comics, however, was even lower than the bottom basement b-movie production house, or even the early days of television. As a result, the doubles, more often than not, were not even remotely close to the body type of the stars they were doubling.
Here is an unfortunate example of a Batman double in a "crushing bubble of doom" sequence from Justice League of America #21 and Mike Sekowsky.