It may be a good idea to give Wilma a can of tuna IN OIL about every two weeks, just in case she's prone to furball. The other main causes of a bloated stomach are: worms, liver problems (especially if the bloating is high up), over-eating (all that food goes somewhere). She may have thyroid problems, but coupled with the massive appetite this doesn't seem to fit the bill. Over active thyroids cause big appetites but weight loss. Underactive thyroids cause weight gain and poor appetite.
Personally I'd go for the easy options first: furball or worms. If treating these fails, a trip to the vet would be wise, just in case she has a tumour.