Jim,
Don't know what to tell you. It could be diet, it could be the strain as there are several strains of perch out there. Interestingly the Lake Erie strain I have doesn't get the huge bellies that I see in local perch out of natural lakes in the area at least not in my pond. Not sure why. So my big perch could have weighed even more had she had the big distended belly I see in local natural lake perch.
If you want to see some real tubbies go to the Michigan Department of Natural Resources site where they keep track of record fish. I would give you the link but for some reason the page is not coming up today. I seem to remember some 14 inch perch that were over 2 lbs.