Yes, it can be bad to oversize a water softener system. If your water softener is too big for your home, it won't do enough work to trigger regeneration on time. When a water softener doesn't regenerate, it's not as effective and bacteria can grow in the tank, and the resin bed can get damaged. So the ability to produce soft water can be hindered. There are a lot of ways to make sure your water softener is properly sized for your home.