The part numbers are different, however the 2 trackpads come with different cables. So it may be that the trackpads are incompatible, or it may be that the trackpads themselves are identical but the cables are different (they route to different locations on the motherboard, I am guessing the connectors are the same though.)
I can now verify that the mid-2209 13 and 15 MBP trackpads are the same, it is only the connector cable that is different between the 2 sizes.
Keep in mind that there was at least 1 generational change in trackpads, the original unibody models (pre mid-2009) had trackpads that were physically different (the little hinge nubs on the bottom of the trackpad are in different places, it's easy to spot once you know what to look for.)
From mid-2009 to 2010 (and maybe later), the 13 and 15 trackpads should all be the same. I'm not sure if Apple has made any changes to recent MBP trackpads though.