Question

why is one of my arms smaller than the other?

Answer 1

Is it?
If it is, it is probably b/c you made/compelled yourself to think that.
Hypnoses, that's what it's called. in this case you did that to your self.
Someone made you really believe that!

Answer 2

We were made asymmetrically, so even though it seems everything on our body is symmetrical, if you look closely you may see differences... :P

Answer 3

It's normal, one arm is always smaller or longer than the other.

Answer 4

It's not uncommon. If you're right handed, it happens naturally all the time.