This probably has to do with the general beefiness of the average extra. Back in the 70s thin was very much in, and there wasn't a lot of weightlifting and bodybuilding outside weightlifting and bodybuilding circles. Big guys like Clint Eastwood were big, but not ripped the way you see actors today. Bulk and muscle are much more a part of the modern cinema male physique. I remember watching Jason and the Argonauts with my kids (granted that was filmed in 63) and thinking "Wow, this dude who is supposed to be the main hero is positively scrawny." (Much like myself, actually). Today if the filmed it the dude would be huge. Don't even get me started on Hercules.
I can't speak to the military--the actual military--of then and now, but guys now are generally bigger than the photos you see of soldiers in Vietnam. Not that there weren't big guys then, but weight training, like everything else, is easier to do now for most, so many many more guys do it.
THis is all me talking out my posterior, but I've noticed the same thing in the troopers of then and now.