2 things that affect drag
The drag coefficient and frontal area, usually abbreviated to one function CdA as for an object you can’t easily change either, i.e. there’s no point quoting Cd for a car, as you can’t shrink it.
So halving the wheel size would halve the area (like a moulton).
Wider rims reduce Cd by flattening the tyre sidewalls so it presents a more streamlined shape rather than narrow rims which pinch the tyre in again at the bead, you’d get a similar (well, even better) effect running 21mm tyres, but they’re uncomfortable and a harder sell to the fatty sportive set.
The reason it makes chuff all difference on an MTB is we’re doing half the speed and drag increases with the square of speed (assuming the reynolds number is high enough, which it will be). So things like rolling resistance of the tyre and it’s ability to roll over stuff is more important.