I think this song means that no matter how big you are or how tough and invulnerable you may seem on the outside, you're true height isn't physical but emotional. By being tall he isn't referring to height but how tall you feel. And by "You're only as small as the world will make you seem" he means that being short isn't a physicality but an emotion. He means letting the world bring you down is the only way to really be short.
I think this song means that no matter how big you are or how tough and invulnerable you may seem on the outside, you're true height isn't physical but emotional. By being tall he isn't referring to height but how tall you feel. And by "You're only as small as the world will make you seem" he means that being short isn't a physicality but an emotion. He means letting the world bring you down is the only way to really be short.