how you store your fat also depends on the proportionality. When I'm 'fat' I have very proportionally small arms. When I diet down my arms are almost out of proportion large. I store ALL my fat in my torso. You might not feel like your arms are so skinny after you diet.
What types of gains are you talking about? If you have never beaten the hell out of your arms before, they will respond better than if you have been for years.