There are five different hitch sizes, called categories. The higher category hitches have sturdier lift arms and larger connector pins.
Category Tractor HP Top Link Pin Diameter Lift Arm Pin Diameter
0 Up to 20 5⁄8 or .625 in 5⁄8 or .625 in
1 20 to 45 3⁄4 or .75 in 7⁄8 or .875 in
2 40 to 100 1 in 1⅛ or 1.125 in
3 80 to 225 1¼ or 1.25 in 1 7⁄16 or 1.4375 in
4 180+ 1 3⁄4 or 1.75 in 2 in
One should note that even though your tractor may have a particular category hitch, that hitch may be rated for more than your tractor can safely lift. What the manufacture states your tractor can lift, should be used as a maximum lift, not as a safe working limit. I have seen many tractors that are rated to lift 1,200 lbs and they are unsafe at 700 lbs!
The difference between one cat size and the next has to do with the capabilities of your tractor not just the pin size, including; HP, Lift ability, traction break out force to list a few. For example implements designed for Cat1 tractors that are put on Cat0 tractors will cause the tractor to work harder and in some cases damage your Cat0 tractor since they weigh too much. If you put a Cat1 implement on a Cat2 tractor, the Cat2 tractor has more HP than the implement is rated for and it could damage your implement.
Always use caution and common sense and if you want to push your tractors limits; check with the manufacture, use counter weights, watch your turns and angles, and please use common sense. It always takes longer to do the job when you end up in the hospital.