Well, a bigger gun does increase firepower indirectly, via its calibre !
By the way, why is there no intermediate howitzer calibre between 105 and 150mm ? 120mm would be perfect for ranged attacks against 40mm plating (like Combat Armor). Right now, you're forced to either go underpowered or overkill...
This is a great suggestion, however there needs to be some additional progression for firepower and costs, if you were to introduce right now a 20mm/5 attacks howitzer from the very start it would both be very expensive to field and absolutely SHRED <7mm plating targets !
So, let's see : right now a 25mm Howitzer costs 20 metal & 15 IP for 0.6 ammo/shot, while a 150mm Howitzer costs 50 Metal and 35 IP for 2.6 ammo / shot - both for a Padded Arty... hmm, now that I think of it, the 150mm with 6 time the calibre, 2 time the attacks and 2 time the attack power actually is waaaay too cheap (even for the ammo cost, which is only 4 times higher) !
And since both the 25mm and 150mm are available from the start, it's no wonder that players directly pick the biggest ! (At least for foot arty, where weight isn't a consideration.)
So, at the very least, bigger calibre Howitzers (that foot arty can use) need to be made much more expensive to field and fire, because there's probably no point in using anything than the biggest 150mm!
(I didn't do the math for the stats of the intermediate calibres, but I guess it works in the same way?)