Back then you didn't have a lot of choice of what language to use, windows was MFC on visual studio and Mac was codewarrior(I think). The Mac and Windows versions were written by two separate teams even though we sat next to each other, and I can't think of any code shared between them, anything we could have shared would have been trivial to write from scratch. I have no explanation for the enormous size of the Mac version.
I'll be damned. Joshua Stephens for real? I know your name from the Eudora Extended Message Services API docs -- though that was long ago. There were like four names there. How much of that was your work :) ?
Very interesting. Were you implementing to a spec with the Windows version, or was it simply just 'make the Windows version do what the Mac version does'?