Don't be too certain that as they get bigger they will be unable to escape!! Mains electric fencing is really the only way to stop them, although you need to check the lower strand, as they are prone to rooting soil all over it, thereby causing the current to short out. Also, I have had one of my sows launch herself clean over average height pig netting. She did kindly squash it down on her way back in, thereby making the fence smaller, and letting the other sow and my best boar who were in with her out to the freedom of my paddocks. Apparently, the three of them get on quite well with the horses, when all grazing the same area. I did get an earful from my other half until I called the three of them, and they came trotting up full speed. The horses, of course, completely ignored me!! Which is the more intelligent animal???