Each individual is different. No 2 persons are one and the same, so the same applies to your cousin. Yes, she must be more powerful than you owing to her figure which is even heavier than you, so why are sulking with this?
It’s no biggie for you to be ashamed just because she defeated you in arm wrestling and she is capable of carrying heavier weights than you. Owing to your small figure I presume you’re no match to her strength, so accept this and get on with your life.
Don’t live under the impression that men should be stronger than women; just see the women engaged in sports or weight lifting, I bet no normal guy can be a match to them. So stop worrying about it.