Why would you increase PGrens? I mean they are already elite and you don't use them to cap stuff
Because PzGrens are no good with PzSchreks. They're designed as an Anti-Infantry squad with the option to get some AT. With 4 models with 2 Schreks, they're too weak to handle any tanks because they don't have the health, and with 4 models with 2 assault rifles, they're not doing enough damage to handle infantry.
Giving them 5 models and lowering the damage for each model's assault rifle so that they're still doing the same damage overall is the best solution imo. You then get to upgrade them with PzSchreks, so they're now a slightly more durable anti-vehicle platform, and then you still keep 3 anti-infantry models which will do more damage to infantry overall than the 2 models did before.
If you don't understand me, I'll make up some numbers and explain;
4 PzGren models do 20 anti-infantry damage. This means 5 damage each. 5x4 = 20
If you increase the amount of models to 5, to get the new value for how much damage they do, you divide the total damage by the number of models. 20/5 = 4 damage.
So now you have 5 models doing 20 damage, 4 damage each.
When you upgrade a 4 model PzGren squad with PzSchreks, you have 10 total anti-infantry damage. 2x5 = 10.
If you upgrade a 5 model PzGren squad with PzSchreks, you have 12 total anti-infantry damage. 3x4 = 12.
So, now you have more health on the squad, which makes them better for fighting tanks, and you now do 20% more damage against infantry when you're upgraded with PzSchreks.