Your question is a bit vague, but I'll try my best to explain.
Load balancers, depending on the implementation, generally just forward requests to the servers. Sometimes they act as a proxy, but that's not how they're usually intended. Think of normal load balancers as a traffic cop that divides the requests between multiple servers.
Proxies do increase the overall number of packets sent across a network for a single unique request, yes. Generally, a proxy is put in a place where it can consolidate requests so that 100 people asking for a specific file do not all have to download it from the original server. The real benefit in this is making a proxy keep a copy of a dynamic page for a set amount of time. Wikipedia has a very good implementation of this, which is partially described on Wikipedia's Meta site
So, in your example, a 100Mb file would generate ~200Mb of total network traffic plus overhead. 100 users, on the other hand, would generate only 10100Mb (100Mb * 101) of traffic, and only 100Mb of it would ever reach the original server.