To check for large roaming profiles, go to the location where they are stored and do a "properties" on the folder.
If the problem is large roaming profiles some of the following may help :
1) Use GPOs to redirect Home Folder, Application Data, Desk Tops and Start Menus to network locations - this should stop large files being downloaded when logging on (especially the desktop!). Redirect anything you can away from "documents & settings", cookies** and Internet Explorer cache are two big offenders.
2) Use a mandatory profile - this would probably be completely unpopular, but it makes support far easier and stop NTUSER.dat from growing in size.
3) Regularily purge all NTUSER.DAT files - these files can become huge. In fact just delete all the profiles, only a temporary measure but it'll speed up in the short term.
** How long does it take to download 20,000 cookies from a remote location to a local computer?