I am running a Tor Cloud EC2 instance. From the log files, it looks like it receives more traffic than it sends. What causes this?
How many users does your cloud bridge have? Many bridges, especially new ones, haven't had their address given out to very many users yet, so they see very little activity.
At the same time, your Tor fetches directory information (list of relays in the network, whether they're up right now, etc) in order to have it ready to give users when they show up.
So it is quite reasonable to have a small bridge that reads more traffic than it sends.
If you have a very popular bridge, you'll be asking the opposite question -- because it will be supplying directory information to bridge users and so writing more than it reads.