The EFSS is a rifled mortar. It is very heavy, well over 1,000 pounds. An 81, in contrast, is man portable.

Similarly, the USMC is banking on computer simulations that show that four M777A2s have enough firepower to replace the six that you would find in an older Battery. Whether this has to do with better fire direction, better target location, improved control with DFCS or even better lethality with M795... that I can't say, but all those things have gotten better since the six cannon battery was standardized.