yulzari
Staff Sergeant
When you make bigger better mortars they have a longer range so their support becomes able to cover beyond the regimental level and need to be allocated between more targets so need to be controlled by a higher level to make best use of them and incorporating them under artillery rather than infantry is logical. The next step is the infantry battalions see a need to retain an organic local area fire support and get 81mm mortars and the cycle repeats. The limit for the battalion level mortar is to be man portable by the crew. The same limitation as the Universal Carrier was built around, ie the load must be made of man portable items.
One reason the British loved their 2 inch mortars was that they stayed with the infantry and no one was going to group them in a higher formation, so the platoon always had indirect fire support and the troops got very skilled with them even with just a painted white line and eyeball judgement to aim them.
Thus we saw three classes. The 2 inch that can be carried complete by one man, the 3 inch that can be broken down into man portable sections and the 4.2inch which has to be carried into position by a vehicle. Or 61, 81 & 120mm if you prefer.