MediaWiki sends a custom X-Vary-Options header on responses. The reason for doing so is described here:
This header helps patched caches. However for those sites which don't have that patch applied, or who don't use Squid at all - i.e. pretty much everyone outside Wikimedia - this feature just adds around 137 uncompressed bytes of useless padding per request.
This header should be optional, and off by default, unless/until this patch is accepted upstream in Squid.
FWIW, I also feel this header should also be stripped out from Wikimedia responses going to end-users (==if there is no Via: header in the request), otherwise you're needlessly sending them ~260 uncompressed bytes per request:
This would presumably involve changing the Squid patch itself.