in 2004 I believe all they had were full size machines.
I think the whole midsize thing started in about 2012.
The difference is mostly that the midsize is about 4 inches narrower so that they will easily fit in the back of a pickup for transport. They also have a different front suspension (struts instead of shocks and dual A arms), the midsize has less payload capacity. I also think it doesn't have the tilt wheel and as much underdash storage space.
The fullsize units of today are the same size as your full size unit minus the newer 900s which are on a completely new and different platform.
Is yours a solid rear axle? I borrowed one once from a friend that was and that was the worst riding machine I think I've ever been in! my back was messed up for days!
the bright side is that your 500 is one of the best engine polaris has ever built. tough as nails and will last a long time if well cared for.
Hopefully this makes some sense.