Why can't we store big objects in session?

[Chris Wahl]

Howdy ,
 
I 'd like to ask a question about session.
It is  "Why can't we store big objects in session?"
 
What I wonder is that sessions reside in Server side and 
are not transfered back and force between Client and Server.
I think they do not cost network band.
 
So why we can't use it to store some objects?