1) I'm not fat.
2) My arms aren't super muscular.
I've been measuring my arms (along with chest, waist etc.) for a few months now, and they sit right at 18 inches. I didn't think that was a big deal until I started looking at a lot of body builders who look massive compared to me and they only have 16.5" arms and crap.
Is it possible I'm measuring my arms completely wrong or something?