They aren’t more muscular than fifty years ago.
The only troops that seem to have gained muscle mass and often look beefed up are American soldiers. This isn’t a new thing, though; it happened somewhere in the 1980s. After the war in Vietnam, the US military got rid of the conscript system and started taking more care of their soldiers.
It was also always a cultural thing: Americans like everything a little bigger than the rest of the world, whether it’s their cars, their houses, or their muscles.
When you visited a US Army installation in the late 1980s, many soldiers were working out and the food was plentiful, tasty, and rich in calories. In comparison, most of the other Western armies had terrible cooking! When I was a paratrooper in the German Army, we were always happy when we got an American meal and not some crappy German, French, or British stuff.
A picture that was taken more than 40 years ago: a Soviet airborne special purpose (Spetsnaz) soldier during the war in Afghanistan (source unknown).
The only troopers that looked beefed up in other countries’ military forces were some of their Special Operations Forces. These guys often got better food than the average soldier and they also hit the gym.
Still, when you look at the German “Kampfschwimmer” (comparable to Navy SEALs) of today or French Foreign Legion paratroopers, you rarely see the “muscular’ guy, but most of these elite soldiers almost look starved.
In combat, every unnecessary weight that you carry around with you is an impediment. You need tough and nimble fighters and not fitness models.