well, I don't know too much, but firstly there is I^2 * R loss. varying the AWG will change the R. So lookup the conductance of copper, and the diameter of the copper wire and I'm sure you can calculate the R.....at DC....
hmm....yeah but transformers are more complicated......
Also there is the fact that for high frequency power transformers, winding many parallel winding of smaller wire diameter performs much
better than a single larger wire winding.....
...even when the total cross-section area of copper of the windings of 2 such comparable transformers are the same......