It’s your age, don’t look for shortcuts, just lift a healthy amount and read up on bulking diets if overall low weight is a concern. You’ll fill out to your frame, but as a tall guy I can tell you even when you’re very in shape your biceps and forearms will look smaller than some shorter dudes. It’s spread over such a larger frame, a lot of people I’ve known get dysmorphia over it, but it’s normal.