They fit people in those things by cramming them on top of each other. Back when the M113 was introduced troops didn’t wear as much kit as they do today and the general American soldier was actually smaller in hight and build (not fat, like some would interpret but muscle). The M113 is basically an aluminum box with benches on either side. This is part of the reason why the M113 has been phasing out of US service as time progressed from the 50s NATO soldiers in general got bigger as their nations food supplies improved. (This can also be seen in the difference between the DPRK and ROK soldiers in which Northern soldiers are generally scrawnier and shorter than their southern cousins.) The other is that the protection needs went up as well.As a humorous aside, on the same day, I remember there was a M113 puttering outside. I was thinking "How do people fit in those things?" It was barely larger than a big van. It looked so stupid to me, lol.