If the help file content is large there is little you'll be able to do. The output type you use may help a little but you'll still have more or less the same amount of actual content. Could you just install the content on the field reps PCs to start with. OK this could be more difficult to update but would get around the download issue. Another option is your developers could compress the output and have a script to decompress it when it arrives at the field reps PC?
Are the users behind a firewall preventing the help from being accessed direct from your server? This would require changes to the help calls.
Would you be able to split the project and produce merged help in such a way that the changes only require downloading just changed outputs? This topic explains how that would work. http://www.grainge.org/pages/authoring/merging_webhelp/merging_reasons.htm
See www.grainge.org for RoboHelp and Authoring tips
I used to have large WebHelp's that presented content that also needed to be downloaded to Field Support Reps' PCs for when they traveled and were unable to reach the intranet.
What I did was to create a CHM version of the content and make it available via a link inside the WebHelp.
Another approach would involve using something like Microsoft SyncToy to copy the WebHelp files from the server to the local PC.
Helpful and Handy Links
Thank you so much for the great ideas. I have forwarded on to my IT support guys, we are having a phone conference tomorrow to discuss solutions. I will let you know how we end up resolving it.