|OK, got it now, it's just that one file that changes its own size when your run it.|
I've not run into this in particular but a virus is capable of changing any file that you run I guess. Mostly viruses attack the more general system files.
Try checking it with MalwareBytes freebie - that often finds and fixes viruses that AV's miss.
I know nothing about foxpro but wonder if the file change is somehow related to foxpro software itself. Might be interesting to see what happens if you make it "read-only".
Please come back and let us know how you got on.