Some aggressive frames may not clear the same tyre on a smaller rim. I have a GT STS-1 with 2in wide rims and 2.35 tyres, and the chainstay fork fully wraps around the tyre and rim, it's keyhole shaped - the same rim and tyre combo cannot be closer to the hub because then they'd foul the fork's stays.

This isn't a common issue, just something to bear in mind and look out for.