They stay small. If you added any plants recently, that is the most likely cause. They lay eggs on the plants, or can even have been on the plants themselves.
They breed rather quickly, so if you have the space I would get a school of small loaches. However, if you like them, just let them breed and don't over feed too much. Your tank is a closed system which means that the amount you feed is the limiting factor in their ecosystem. Feed too much, too many snails. Feed just right, just the right amount of snails. Too many snails can screw your bioload, but having them just right is beneficial. It adds interest to the tank as well as a clean up crew that is self propagating.
