You are only thinking about the limit of your total army. The whole point of the popcap system, is the upkeep you are paying for it. Since you don't have ways to increase your man power, every point on pop is going to hurt you passively if you don't make use out of it.
Oh right, CoH's strategic points also gave you +3 manpower if I'm not mistaken apart from 3 more pop cap slots until you reached 125 which was the max.
If you make one model = pop size 1
-->
5 Conscripts squads-> 30 Pop
5 Grenadiers squads -> 20 Pop
Which means that the Soviets pay crazy upkeep and cant field enough units. Since one conscript model is significantly worse than one Grenadier model. Same for weapon teams.
That´s the reason why COH2 doesnt use the one model = 1 pop size. Thankfully.
Just imagine how unbalanced the game would be if lets say both sides had 5 mainline infantry units + one mg + at gun + mortar.... Soviets would end up at 48. Ost at 32.
Yeah yeah it makes sense now that I remember about the upkeep being tied to pop cap.