Mike,
Chris is right about the jpeg compression changes. Any difference in the photo will give different compression requirements and affect the final file size. Also, since I presume you're shooting straight jpegs, that camera has a lot of different file size and aspect ratio choices. If you've played with any of the settings between shots (which I doubt since they're buried in the options menus) that could do it too, although I'm not sure the stack would work if you did that. If you shoot raw, all the files will be the same size. Bigger, but all the same. You can convert the raws to tiffs to stack in ZS. I almost never use jpegs and sRGB color unless I'm emailing or posting them. But that's just me. I have an issue with spending a ton of money on a nice camera sensor then trashing a huge amount of image data before I even save the file!

I'd rather process the file first, get what I want, then convert to tiff/ProPhoto RGB or jpeg/sRGB. Admitedly though, that might be overkill for this kind of stacking.
Tom