When you are connected to the Gnutella network, searches are going into and out of your computer. A lot of traffic comes downstream to you. LimeWire will pass on the search requests from others and will also send out results for search requests that match files in your library. This Gnutella traffics can easily consume all of you bandwidth (up and down) if you have lots of connections. Dial-up users can't usuall handle more than 3 at a time. Soooooo.... even though you have disabled file sharing, you are still an active member in Gnutella network activity, and you will therefore use a lot of upload bandwidth to participate in the network.
:-)