Wedding portions are a little smaller because it is assumed the cake will be served after a full meal and drinks. It is also a good serving standard for cake sizes. The standard is 1x2x4 inches as KoryAk said.
Party size is generally considered for events where cake is the only food item. Birthday parties come to mind. The standard is 1.5x2x4 inches like KoryAK said.
It is a guide that depends on what is being served at the event, not the event itself. BTW those are good sizes. Prove it to yourself by cutting a piece of foam or even a couple of slices of bread to see how big it is.
Bigger sizes are serious cake eaters and should be used to buying/paying for more.