6 Mb of photos take up 51 Mb on iPod nano

I've got 92 photos taking up about 6 Mb of hard disk space on my PowerBook that I also store on my 4GB iPod nano. The problem is that iTunes will not sync all of these photos to my iPod unless there is at least 51 Mb of free space on the nano. I have tried a software restore on the nano, but this did not fix the problem. The Photos folder on the nano is only 4.4 Mb in size, so I don't see why that much free space should be necessary.