A couple of things come to mind here. I dont know what the disc sizes are, meaning, if say the windows machine was an old xp machine with like a 30 gig drive and the mac is newer and thus has a significantly larger drive (1tb or so) you will get a difference because of the way the file system allocates data.
Also the file systems are different so each file system will write the data differently ( hope I'm saying that right)size will be different this will add up more than you think.
Kinda think of this like when u make a backup onto cd the cd file system is much more efficient, so the file is "tighter" normally than off of your hard drive.
However it does seem like a large gap I would def question it myself if I were in your shoes. Have you compared the two to make certain there are the same number of files in each?
I have seen that there are many people here significantly "smarter" than I hopefully one of them will chime in.
good luck