How does your tab fit?
I have a similar issue but my callous, albeit minor, is due to my tab being just a shade to small. When I'm at anchor my ring finger is relaxed almost to the point of not being on the string at all (the majority of the weight held on the middle finger with the index taking up the rest of the slack).
Doing so causes the bottom half of my ring finger to "ride the edge" of the tab and as a result a small callous has built up. It's not painful and there's no numbness in the digit so I'm not overly concerned about it. I suppose I could switch tabs but I have yet to find another one that I like so for now I'll stick with the rough patch on my finger!