Why does a Tor relay send more data than it receives?

I run a Tor relay. I have often seen that there is a significant discrepancy between the amount of data sent and that received. For instance, this morning log states:

Tor's uptime is 7 days 18:00 hours, with 551 circuits open. I've sent 167.80 GB and received 140.35 GB. 

That's a nearly 20% difference. I had a view of a relay as little more than a passing station on the road, except for the occasional authentication to the network.

Does anyone know the origin and meaning of this discrepancy? By the way, I am not hosting any hidden services.

Solution 1:

In addition to the link about being a directory service.

ToR adds an additional layer of encryption each hop a request goes through, that would mean that outbound traffic would be larger than the original request.

TOR instead uses, at a minimum, three servers to pass your traffic on, each encapsulated with its own layer of encryption.

http://www.techrepublic.com/blog/it-security/everything-you-need-to-know-about-using-tor/