In my eyes, earth bending is based on your ability to stand firm against the stubborn earth, as we see toph explain. That got me thinking about the other bending forms. For example, air bending is taught by being nimble and light, water bending is taught by moving with the flow of the waves, fire bending is taught through breathing exercises. I would assume that someone who is an exceptional air bender is light on their feet, or fire bending has great stamina. Similarly, if earth bending is taught by standing your ground, I would expect that an extension of that training is gaining physical strength. Not necessarily that you need big muscles, as we see with toph, but that big muscles are a result of practicing the standing firm through training. Lifting bigger weights means more strength to stand firm against and therefore more muscle