As with many such questions: a lot of hidden variables. Enough to keep several scientists employed for many years. I would suggest looking at this paper as a start, also the work of Charlie Ellington.
R. Buchwald and R. Dudley
Limits to vertical force and power production in bumblebees (Hymenoptera: Bombus impatiens)
The Journal of Experimental Biology 213, 426-432 © 2010.