Can they be "too slender"? I guess so, but like Jim and Tom are talking about, I think everything is tied into architecture. I know it's an area I struggle with constantly, and I think a fair number of contemporary builders tend to make guns a little "thick".
The one thing I'm most often struck by when I examine original rifles, is their slender, almost delicate aspects. Not all are that way, especially earlier guns, but even they have an almost petite nature to them.
Ed