I am (very) new to Tor.
As I understand it, each time a data-package passes a Tor-relay, it is encrypted/decrypted, thereby changing in size only marginally. It seems to me that if somebody monitors the Tor-relays, he could analyze the size of the data-packages entering/leaving the relays. Combine this with an analysis of the timing of each data-package entering/leaving a Tor-relay, and you should get a very clear picture of the entire path, revealing each individual Tor-user and his activity-profile.
To avoid this, all data-packages must be chopped/padded at every station after encryption/decryption, so that they all have the same size en-route. Also, to avoid time-analysis, a fair number of bogus data-packages must be swapped around the network, so each Tor-relay and indeed each Tor-user, appears to have a constant workload.