smart:EQ 4 extracts a lot of information during learning from your audio files. When storing a session, this information needs to be stored too - otherwise, smart:EQ 4 wouldn't be able to compute a new result when e.g. switching profiles in a restored session.
Depending on the number of smart:EQ 4 instances and the learning time used to compute the smart:filter (the longer the learning time, the more information needs to be stored), the size of your session file will increase.