Joe,
My understanding of the terms narrow and large is that they relate to width, rather than length.
AXYs have the same size of outer shell as the single coil pickups on the Series instruments, but inside there's a smaller magnet, with two coils in humcancelling mode.
MXYs are exactly the same pickups as the AXYs, but the shell is narrower.
Fatboys are AXYs that use the large magnet from the single coils - so they are different pickups in identical shells, where MXYs and AXYs are identical pickups in different shells.
FatBoys are the same size as AXYs, and both are significantly wider than MXYs. So you would need to enlarge the original MXY routings to make an AXY or a FatBoy fit.
IIRC, they have made a small number of wide aperture MXYs, to get a similar effect to FatBoys - not sure if they would still make them.