Thats not a dumb question at all.
I know theres the common misunderstanding that rolling radius are the same, i dont know exactly where it comes from, proably due to fact they're within a percentage of difference that makes the swap legal.
Anyhow thats incorrect, get wheels of different diameter and have a look youself. They're accutally bigger!!
If you make the maths it will turn out that:
A 12" wheel will make the mini sit about 0.7" higher
A 13" wheel will make the mini about 0.8" inch higher
(Tyre pressure will play his role though.)
PS:
Heres a compare-o rama for tyres, saves us to make all that maths for such a thing 
http://www.mintylamb.../?page=tyre.htm
If the radius of the wheel + tyre is the same though (as in exactly the same), then the centre of the wheel will be at the same point, regardless of wheel size.
Obviously the difference between the 10s and 13s etc is such that there isn't a tyre which can give an exact match for rolling radius, thus altering the centre point of the wheel and the ride height.
Not sure as there's anything legal to do with the rolling radius stuff, it just means the closer you can get, the more accurate your speedo will be without being recalibrated to the new wheel diameter.