Actually, the Sweetums costume in MV3D isn't bad at all - but you're right, the Kermit and Piggy costumes aren't very good. No Kermit and Piggy costumes are - they aren't meant to be seen that size - Kermit will always be too fat because human beings are not built the same way Kermit is. Same goes with Piggy, the dimensions just won't be right. But Sweetums is at his correct size, so proportionally he's correct. So he looks okay. I just ran into some pictures from the Muppet stage show in MGM studios from when I was in Disney World in 1991 or so - I'll see if I can post them here to show how wrong they were!
Jeremy