>>64283547
They do. Most gundam happens in basically a single decade. So design is just how much cool shit can we stick onto the basic idea. When you have an actual gap in time. You get F91 which discusses how major advances in tech have allowed for smaller suits being just as capable.
The increased size within the main decade is commonly down to defensive gear like iField generators, and beam weapons. Basically the zaku 2 is a walking tank. The gundam is an extremely expensive prototype walking tank with a battleships energy canon. Mobile suits chases the energy required to also wield energy weapons without the gundams cost. And so got bigger. Then as energy weapons become ubiquitous the only defense again them iFields become popular and that takes even more power. Like compare WW2 destroyers to modern destroyers. An Arleigh burke is almost the size of WW2 cruiser. But then we get a big 30 year gap and now mobile suits are smaller than a mobile armor with twice the capabilities.