In any case it is always better to use SSL-encryption – also in your own network. You never know who is eavesdropping. It is easy to spy on you if you use WI-FI.

I suppose most data consumption comes from images. Even avatars can be quite big, yours e.g. is 192K which is quite large given that it is displayed only as a little icon in most cases. That's the downside, when everybody can provide their own images and the server does not downsize it accordingly.