For many reasons, the size of modern armies are relatively smaller compared to even 20th century armies. In my setting there is a more infantry centric style of warfare, but one that's set in near future.
Now the problem is that modern militaries are highly optimized to kill effectively from range. This also means that militaries don't need to field large armies. Tanks are a major force multiplier, even IFV/AFVs are. However, in my setting there is not only power armor, but also shielding for said power armor (its frontal, top, and side only). The shielding is incredibly strong, but its expense in shield matter and power requirements scales exponentially for larger machines. So, the same technology isn't applied to vehicles on any scale (there are some other quirks that make less useable for vehicles in general, movement speed being one of them). There is also the mass proliferation of drone technology and far more reliable anti-tank systems. A weapon the size of older tripod recoilless rifles is capable of firing armor defeating rounds, top attack weapons, loitering munitions/drones, far deadlier anti-tank launchers exist (think Javelin/NLAW size) than our current time period. Tanks, and IFV/AFVs are essentially relying on active protection systems, experimental armor alloys, reactive armor etc to stay alive on the battlefield. Infantry units on the other hand have gotten much more survivable with the existence of troop sized active protection systems, counter suicide drones the size of a water bottle, plus their shielding and more.
As such vehicles are used more as they once were in say WW2 or Vietnam. Direct fire infantry support. But they are very vulnerable. Think of it like the early interwar years between WW1 and WW2 where countries were fielding reasonable anti-tank rifles before tougher armored tanks appeared. Maneuver warfare isn't as strong as it was say in Operation Desert Storm or Operation Iraqi Freedom. With a higher degree of importance placed on infantry, would one expect to see army sized increase in size, decrease or stay the same?
Power armor in this case is simply more about hauling the power source and shield "generator", as well as allowing infantry to carry larger weapons. The armor itself, outside of the shield, isn't anything more than structural metal and what is essentially a flak jacket. So, it's no iron man or 40K Space Marine. While it can't shrug off a direct round from a tank or cannon armed IFV. A tank round that misses and digs up dirt isn't going to kill the soldier outright. They'll be knocked around and their particle shield might run out of particles trying to keep them alive, but the soldier will live. And if they're in good condition, they can get another container to fill up their "shields" again relatively soon.