This is the same old debate, but would you ever actually go buy a tiller that was not as wide as your rear wheels???
When the average person tills their garden, they expect to see a seedbed of tilled ground from edge-to-edge. The entire area. Tilled up. Not with untilled paths in the middle. Anyone paying to have their garden tilled expects it to all be tilled and smooth with NO tire tracks or paths left.
If you want to leave untilled areas, you can certainly do that with a properly sized tiller. Just move over.
On the other hand, with a narrow tiller on a wider tractor, there is NO WAY to leave an entire area tilled with no tire tracks.
So how does it make sense NOT to have the tiller a little wider than your tires? Seems silly to suggest that it's handy to have a too-small tiller. Worse than too big.