NTUSER.DAT conatains all your user settings e.g. which background to use, what software settings you use, items pinned to the start menu etc.
To keep this down why not try renaming the default ntuser.dat file to ntuser.man. This will stop the file size growing.
The only problem is that this won't retain any personalised settings. It will keep you working with the standard default user profile at all times even when the user changes a setting. It is almost like a read only profile.
I haven't fully tested this yet but it may also be useful to prevent non administrator users installing certain per user applications too, as not only then would they not be able to write to HKEY_LOCAL_MACHINE but in theory they couldnt write to HKEY_CURRENT_USER. And even if the software does install most likely it wouldn't work once the registry edits that the setup has made have been deleted.
Hope this helps, not sure if it would suit all environments though.