It could be due to comparing "Size on disk" instead of filesize. The filesize is the exact number of bytes needed whereas size on disk is how much space of a disk drive is required. The descrepancy arrises when you have two disk drives with different sector sizes.
(example: a 513 byte file on a disk with a 512 byte sector would require two sectors and take up 1024 bytes on disk. That same file on a 4096 byte sector would take up the entire 4096 bytes on disk. It doesn't seem like much difference but if you have thousands of files, they add up.)
Use the Get Info command in the Finder for each copy of the library and you'll see the size of the library and also the size on disk.
If you're looking at the size in a Finder window I believe it is showing "size on disk".
This is only a theory. See if it helps.
does your external drive have the same file system? Is it formatted "MacOS X Extended (Journaled)"?
If not, I'd reformat the drive, before you copy the the library there. An Aperture library will not work properly on a drive not formatted for mac.
You'll see the formatting in the Info panel of the drive when you select the drive in the Finder and use the command "File > Get Info".
yes - I only use a Mac and every time I format I do so as you suggested [extended-Journaled] - I think I might have figured it out == not sure yet but as you can see, library 1 is different from 2 but one version is diff. from the other = I'm mainly moving older aplibraries to a backup drive and looks like I should open lst and then move = not sure that's always been the reason but I'll do some updating of the older ones and try again...
Guessing -- -- I suspect the difference has to do with how OS X copies Finder packages. The Library is a Finder Package. Finder Packages are collections of Finder Folders and Files which show in Finder as a single file.
What happens if you copy your Library from one drive to another drive, and then copy it back? Is there any reported change in size between the original and the "copied-over-and-back" one?
In the past one could force a stringent defragmentation of the Finder Package by copying to a different drive (the OS copied by file and rearranged the sectors).
I haven't done that yet but did speak to Apple with someone versed in Aperture == he thought it might be similar to what Kirby suggests -- there are data within the library that are specific to its location [ie caches and other information that has to do with the specific drive it is on, ie my MacPro] but when it's moved, that information is not relevant so there are fewer bytes. We looked at the two libraries and they have the same # of pictures so it may be some deep internal bytes 'missing' == these are not drop dead Libraries [just backing up so I can trash ones not worth working on, finally] so if there is not much difference, I'll not worry about it == if it's an important Library I'll compare the # of images....Ultimately it will take too much time to be totally rigid about it and I'm not in possession of the time or patience, but at least I now know what may be going on and what I can do to double check before I move on. Thanks much. By the way, these are not [for the most part] referenced images = I tried that, got helplessly lost and decided to do managed images henceforth, and was much happier.