"I see a little bit more than twice as much inbound than outbound connections on my (non-exit, non-guard) relay [0]."
"looking at the graphs in atlas (as well in arm) shows no significant (= something like twice as much) difference between the inbound and outbound traffic"
I'm not sure if you mean the literal number of connections, or if you're talking about bandwidth utilization. In either case, if arm isn't showing the increase, the additional traffic might not be coming from Tor.Â
Have you considered other services running on the host?
You could always take a look at the traffic. You could use something like 'iftop' on Linux (turn on the port display) to get a really quick picture.