Sounds like they are downloading a newer version of Reader and not installing it. The latest and greatest version of Reader is Reader X. It has important security features added to it. I'd recommend you installing it system-wide.
The payload for the 9.3.4 to 9.4.1 auto update of Adobe Reader is only 3.3 MB, so that can't be it.
Users attempting to upgrade from Adobe Reader 9 to Adobe Reader X will see a payload of around 35 MB. Usually it runs in the download manager browser plugin; however if they're prevented from installing new software, they won't be able to do anything with it.
If your users have other Adobe products installed (Creative Suite, Acrobat, etc.) these all have independent upgrade cycles, but enterprise installs tend to disable the Adobe Update manager service so they can deploy via GP. It would of course help to know the precise URLs you have logged, as without that we're looking for a needle in a needle factory.
Possibly a few PDF's with asian fonts circulate in your company.
The optional font packages the Reader downloads to display those files can have umpteen of megabytes.
I have Reader 9.3.4 installed and as per regular updates from adobe, I have received 52 MB's patch to get to the latest Reader 942
After reading your problem, I have couple of questions,
1. If the downloads are coming from www.adobe.com then it does not completely confirm that the these are all Reader downloads. Would like to know what others Adobe Products are installed at your enterprise ? May be the regular updates for all these softwares are increasing the downloads.
2. What is the per user download ? and how many users are affected by this. Make your equation with 52 MB's patch size and see if the pattern is expected.
3. If possible, could you also share the complete link for the downloads because www.adobe.com does not signify anything specific.