Flint, they don't necessarily have to be "big and heavy" (although it certainly helps when trying to make pattern-welded steel!). A lot of it has to do with how it's mounted. A smaller one also has the advantage that it can be moved (relatively) easily. I have a couple of 56 lb anvils, and I keep one on the end of my bench so that I can hit things on it instead of using the vise. (My bigger ones are too far away, in another shed).