I'm wondering how your arms got muscular. Did you do a lot of work to make them look muscular? Or are they just bigger than you care to have them look?
As I understand it, if you don't work the muscles, they won't get bigger, and they'll atrophy. If it's girth that you don't care too see, then you have to lose more weight, but if your genes tell your body to carry more tissue in your arms, there's no amount of dieting that will melt that off before other parts of your body. No such thing as spot reduction, outside of liposuction.