Two main differences between the B7400 and B7500. First the engine is slightly bigger on the B7500, and the B7500 also has power steering.
When you go up to the B2410, again the engine gets a little larger than the B7500, and it also gets position control. Roughly the same size frame though (maybe even the same frame judging by published dimensions).
The B2710 and B2910 get a little bit bigger frame than the B7500 and B2410. Each also has a bit larger engine; 27 HP for the 2710, and 30 HP for the 2910. The 2910 is the "premium" of the B series, and it also has a few more bells and whistles than any of the other B series.