Got Squid installed? That's notorious for this kind of stuff, especially since stuff like Windows Updates and other services use HTTP, making Squid want to cache ridiculous amounts of data.
One of the issues has been HTTP request ranges. Sometimes a service may have something like an 8GiB file, but depending on what your system needs, only a small multi MiB range will be requested. But Squid will download the entire 8GiB, then return the 8MiB chunk to you. Not sure if this still applies, but it has in the far past.