It's probably due to the format differences between the machines. On the PC, you're formatting as FAT32 or HTFS, and on the Mac, you're formatting as HSF+ Journaled/Unjournaled or UFS. Each different format takes up different amounts of space on the hard drive. HFS+ (the default Macintosh format) may take about 16Mb on a 128MB drive, where FAT32 or NTFS (the default Windows formats) may take only 4MB.
OS X can read and write to FAT32-formatted disks, so you may want to settle on that format for cross-platform compatibility.