One additional suggestion if do want Norm suggests and that is remove the spark plug so your not fighting the engine's compression. I do know some mowers here a too tight of belt will overload the engine especially when the engine is small for the deck size.
For belts coming off make sure all your belt retainers are in place and that normal alignment is maintained. Now if this is a hybrid then you will have problems until you find the correct belt alignment position may take changing drive pulley and or additional belt retainers. Here my Bolens 38" cut if weren't for the belt retainers the belt would fall off every time I disengage the deck as the belt is that loose in disengage position. If I put on a belt that is only an 1/2" shorter it overloads the engine.
As belts sizes you went down one size are you referring that you dropped a full inch or a fractional inch? An inch can be a lot depending on the belt size.
As for binds as Norm refers to while checking things check the idler pulleys and spindle(s) bearings for rough spots as these will cause binding under load. I have idlers with only rough spot to hang under heavy belt loads and snap belts.
