I tend to take a historical slant, so thinking mainly of Iwo and Malta - but you would be correct where A&A powers are all treated equally and have the same capabilities. Its a house rule - adjust and use as you wish. I do like your China comment - will put some thought into that.
Another way to depict Iwo and Malta higher defense per soldier implied is simply that a single Inf unit represent pretty much less soldiers compared to European land units.
So this Inf worth the same combat value than 10x soldiers per unit in Russian battle field.
So no need to change from OOB.