It seems like the expenditure of resources against the Volm and Humanity cannot possibly be worth any potential gains against their primary foe.
It does seem like their attempt at taking Earth is draining their resources faster than what their previous conquests might've gained them. The very fact that they've stepped up on the offense against humanity suggests that the efforts made by the 2nd Mass, the Volm and other human resistance groups are taking more of a toll than the Fishheads are letting on.
didn't Cochise say (or was it the rebel skitter) that they weren't expecting this level of resistance from humans? They haven't had this level of resistance from many other races. Dealing with this has required more resources and approaches than they had planned for.