Especially for large quantities of hidden data, you might need many containers. For a party that has access to the larger data pipes, it is possible to compile statistical information on the email patterns and signal any significant change. We do not have to limit ourselves to email. Traffic analysis has been used in military intelligence for a long time. By observing the paths of supply trucks or triangulating radio transmitters, the relocation of the military bases can be inferred.
Cellular companies use it to detect fraud by flagging unusual call patterns.
Banks analyze electronic payments to detect buying patterns. Intrusion detection systems do it to signal unusual packets. Of course, there will be a lot of false positives. By itself, traffic analysis cannot tell us more, but that's not its job. Traffic analysis is a statistical tool and thus requires large amounts of data.