I find that the brassica types like broccoli, kale, cabbage etc can make them a bit smellier. They do need some for the nutrients though. During the summer, I feed grass instead of most veggies, and I noticed they were less smelly then. (You have to introduce it gradually though as too much too soon can upset their tummies)
There's a thread on diet in the forums tab which might be useful. Sorry I can't link on this device.
Is it boars you have? When boars are frisky they can get a bit pongy. In general through piggies don't smell too bad.