Check out the Gecko site... there is a lot of information there on matching the components. In particular, this pagehttp://www.geckodrive.com/support.html
is a treasure trove of information. There is a formula (REALLY badly formatted) about 2/3 of the way down that SHOULD read
V power supply in volts = 32 x sqrt( Winding Inductance in milli-Henrys )
My guess is that the strong motors have high inductance, and therefore require a higher power supply voltage to get optimal performance. With 48V, the optimal inductance is only 1.2mH.