The arms actually are fairly thick, just not "fat" or "muscular" thick. The problem isn't in the model geometry, it's in the lack of a muscular normal map.
It's been discovered that there is a way to make the body weight slider affect the texture set used on the body and not just a geometry morph; not some kludge or something, it's inherently in the engine but unused in the vanilla game (I blame 11/11/11). Form lists are involved (which are already referred to in-game). That's how I have the Sporty normal maps apply only to certain weight slider values (with 100$ being one of them, natch).
Now if we could only get a modeler to make a good non-ultra-ripped-steroidial-zero-bodyfat obspace normal map for male characters so 0% weight slider male characters don't look... gross. I managed to kindasorta scrub out the veins, but that's it...
If you're looking for the muscular normal maps for female characters, it's not suitable to link to on this board. I can point you in its direction however: search for "sporty" on SkyNex.
A question to the OPer: Do you have a female body mod installed? I find those to have unacceptably low variance in arm size between 0% and 100%. Along with absurd figure proportions, it's the reason I still stick with a derivative of the vanilla body model.
(Yes, I'm aware of BodySlide, no the arms still aren't muscular enough. I can make them quite chubby, bot no deltoids or biceps or whatnot.)