Joe,
The proportions that you are using are causing the problems. If you take a stock airplane from G2 (the CAP 232 for example), and increase the size of the aiframe without changing the size of the landing gear and wheels proportionally (the same percentage you used for the rest of the plane) you wind up with the problem you describe. You will also need to move the gear and change the CG a little. Try increasing the wheel and gear size and see what that does and go from there.
Dan