Looking at the advertised bandwidth vs bandwidth history from Tor Metrics[1], it appears that guard relays see much more traffic than exit relays. I think it might be partially because guard-only, guard-middle and guard-exits aren't separated, but would it really skew the numbers that much?
[1]
http://rougmnvswfsmd4dq.onion/bandwidth-flags.html--
Finding information, passing it along. ～SuperSluether