Funny, the wgr21 has more explosive power than the 75mm?
Uh.. yes.
WGr 21:
Total weight: 112.5kg, warhead alone 40.8kg
75mm T13E1:
Shell weight (M48 HE): 14.92 lbs, 1.47 lbs of TNT in the shell.
If I have the right shell, it is this:
Diameter isn't the only consideration.
EDIT: Also I believe WGr 21 refers to 21 centimeters. Not mm.