As you can see HTC HD2 is only slightly bigger than iPhone but display size of HTC HD2 is much much bigger than display of iPhone (4.3 inch compared to 3.5 inch).
HTC HD2 has a camera with Flash but camera of iPhone is not protruding at all:
Compared to HTC Diamond, HTC HD2 is huge:
Compared to HTC Universal (the first phone ever powered by Windows Mobile 5.0 and the first Pocket PC with 3G support), HTC HD2 is small:
So, as you can see above, size of HTC HD2 is not that big and we can even comfortably use only one hand to handle it and to use virtual keyboard.