"Each year I am bigger."
Translation:Cada año yo estoy más grande.
I'm confused as to why this would use estar rather than ser. In any other sentence I've seen thus far, physical traits like size are indicated with ser, e.g. "Soy alto" rather than "Estoy alto". What is the rule that would tell me it should be "estoy más grande" rather than "soy más grande"?
You are not stating your size here. You are stating that you are growing. That is a continuous action that is usually stated with the verb "estar". If you said you were tall it would just be "soy alto". With the right context both "estoy más grande" or "soy más grande" can be correct. If you said you were bigger than your friend that would use "ser". Soy más grande que mi amigo.