Doepfer

Hello group. I'm a bit baffled or naive by the distribution of power in the two different cases. I'm thinking of upgrading and just wondered if I need to be more selective and calculating when adding modules?
Up until now i have had 2 G6 cases, and within them I have had various arrangement of my 90% Doepfer modules.
I know the upper limit of my power supply PSU2 is 1200ma rating. 
Perhaps I am overthinking this but how does the larger case with the third busboard still support more modules?
I feel if I understand this a little better I may lean in one direction versus another. 
Any tips ,rules of thumb, or advice would be useful to me. Thank you. 

Raul Pena