Well see if you dont limit filesizes or anything and allow large uploads these types of things can occur.
If someone is for example trying to zip like 24 1mb files then maybe this happens whereas if they are zipping 24 250k files it would not.
It is also different server to server how things are handled.
vbulletin 4 has a very basic profile album feature from what I here same as vb
They are not releasing a photo gallery and as such I do not beleive any zip and download functionality.
Ultimately I think the best solution going forward if you do not limit sizes is to limit sizes or at the very least limit the max dimensions to maybe 1024 by 768 so all photos are truly processed by the image processor not just big files being dumped on to the server.
Example being on my site a 2.33mb file processed with those dimension restrictions is resized down to 781kb so smaller files will be smaller like I have had 1mb files sized down to 140K using 70quality which is the default. My example of the 2.33 file is with 100 quality. at 70quality it is 133kb