jerber: If you are not doing so, you need to give your pigs unlimmited grass hay and cut back on fresh veggies a tad. Pigs need to have the hay or their teeth get overgrown and they end up with digestive problems. Hay is not fattening and neither are veggies for that matter. It's more a matter of nutrition and exercise that keeps pigs from getting fat.
Make sure you give them hay at all times and replace any that gets urinated or pooped on daily. Pigs will not willingly eat soiled hay unless they are starving, so even if you have hay left in the cage, don't force them to eat it. It's how they keep themselves from getting sick.
Not only does it help them physically, it's also a source of stress reduction. As chewing on it reduces tension.
Guinea Lynx :: A Medical and Care Guide for Your Guinea Pig go here and read up on the care guide on guinea pig diet.
The preferred method of feeding veggies is in small meals rather than a single pile. My boys get veggie treats all day. I hand them to them rather than pile them up. It gets my boys used to coming up to me for food and has tamed them very quickly. They even know the names of which veggies are which. But that's another thread.