Does a wider magnet mean less flux density?
Yes, all other things being equal, a smaller diameter magnet has a higher surface Gauss value.

Explaining why could go all the way into the mysterious depths of magnet physics, so I'll just give the first couple of layers.

A theoretical N52 magnet has a "Maximum Energy Product" (peak product of magnetic flux density and magnetic field strength) of 52 MGOe (million Gauss Oersted). The resulting surface Gauss (magnetic flux density) in an open circuit (eg. when the magnetic is in free space) depends on the length to diameter ratio. Roughly speaking, that's because the magnetic field lines spread out in order to meet the opposite pole. The longer the magnet the tighter the lines need to be (higher flux) but the wider the magnet the less tight the lines need to be (lower flux).

So by decreasing the diameter of a magnet with the same MGOe rating, you concentrate the field into a smaller region and register a higher Gauss value at the surface.

This is really tricky stuff that I've only scratched the surface of, so there's some great further reading here.