Well kids it's not if the motor can turn the dish but several other things to consider, things like wind-loading, (as mentioned by RimaNTSS) and starting torque, (as mentioned by Channel Hopper) but the main thing is the weight of the dish in a downward direction, 15 Kg is about the max on most Diseqc motors, this is the combination of the dish, it's mount and the LNB, the downward torque on the motors elevation adjustment is seriously stressed when you go beyond the 15 Kg limit.
And most of the lower cost motors use a plastic gear to move the dish, the newer SG9120 motors have a metal gear, so stripping out with a heavier dish is not a problem, but again the main problems would be the extra stress on the EL adjustment locking bolts, too big (or too heavy) a dish and the added mechanical advantage would/could exceed the design limits of the mount, adding ice or snow to this and it could cause the dish to drop.
The main dish size spec on most motors is 1.2 meters, an extra .3 meters may not sound like much but will it put it beyond the 15 Kg weight limits on most?
This is why the bigger "C" band dishes use a polar mount, this type of mount transfers the main weight of the big dish directly down to the mounting pole, thus saving the motor from having to hold and move the main load by it's self.