It really depends. If you want low down torque etc, bigger is worse. Smaller carbs work better at slower speeds. but then, if you're after wide open type application, ie motard, then bigger can be better. but you have to have the motor to support it ie. porting, valves, cam etc.etc.
Having said all this, my motard's gears is stuffed. I have 4 gears and only ever use two of them! 3rd and 4th. I NEED the low down because I odn't have the gearing to keep the engine spinning up high where a large carb would want it to.
Because of this, a 25 seems to work well. I've heard of the chapionship winning motard bike in MMMCI running a 24mm flat slide, but then I don't really believe much of what that guy says. Motor racing is 99% BS, 1% real. Learnt that from racing cars.
If you could get a gear box to keep the bike running where it wants to, then bigger would be better. 30mm MAY be over kill in some situations, but in other it might work very well. The guy who runs the VSMR said that if he was getting back into it, he'd run a 30mm carb but have choke inserts to lower it to a 26mm, 28mm etc. to suit the conditions and applications.
Food for thought