This is a late one too, but try putting the tire on a standard rim. The heavy duty rims are usually a little wider than standard rims. If you put a wider tire on a thinner rim, it tends to round up in shape. If you put that same tire on a wider rim it has the room to spread out wider horizontally.
I discovered this by putting standard tires on a heavy duty rim and noticing those same tires didn't fit anymore. I had 2.25" tires and standard rims on a '74 Schwinn Speedster. The tires went on the rims and into the frame just fine. I rode the bike for about 100 miles, and even had a couple of thugs try to bike-jack me on it. Later I bought some (non-Schwinn brand) heavy duty rims and put the same 2.25" tires on it that were on it before. These rims were much wider than the standard rims. After inflating the 2.25" tires they rubbed the frame like crazy. Then I tried 2.10" tires with heavy duty rims. They still rubbed the frame. I ended up with 1.95" tires on the heavy duty rims, and to be honest, they look just like the 2.25" tires on standard rims, except this time they didn't rub the frame, but just barely.
Try a thinner tire on a HD rim, you should end up with the same look.
