It all depends how the rest of the bike is set up TBH. For example, a 21" could make the bike slower steering but if the trail is altered to suit than that's almost negated............
There is a bit more of a gyroscopic inertia which, when compared to 17" wheels, makes a bike need a little bit more input to change direction and it can contribute to the bike feeling a bit more 'planted' BUT with the wider bars of a dirt bike you can be hard pushed to notice that.
I think the issue, if there is one, is the smaller contact patch of a skinnier front wheel but with modern tyres this is mostly in the rider's head. The 21" front end on my 1190R seems to cope just fine with the KTM's 150bhp and my ham fisted riding
Andres