Generally, the bag will be fuller, and warmer, in the morning. As the jenny gets on with her day, the bag will get smaller until it is noticably smaller at night. If I remember right, it has to do with how much they are up and moving around. The more active they are, the more they will re-absorb some of the milk. Keep a close eye on the jenny who has a bigger bag at night than she did in the morning. It might mean something is happening with that baby!