The list generates a single digest to be sent to all subscribers who have subscribed to the digest, so it is impossible to customize it for different subscribers. The list automatically sends the digest when it reaches 60k in size, which tends to happen every couple of hours during the day. There current tends to be about 5-10 digests a day.
I get a small stream of emails asking about adjusting the size of the digest (normally to make it bigger so less digests are sent). If it is set any bigger, then some recipients will start having trouble (in particular those getting it on Blackberries) receiving the digest because it will be too big.
While the digest is a popular form in which to receive the list -- it has been chosen by about half of the subscribers -- It also has some significant drawbacks, and the solution to too many digests may actually be to go to individual emails and a filter. See There are too many messages!! for a full discussion on this, and hints to handle it.