Rebels have always had it a bit harder to activate as many squadrons in a single activation as the Imperials.
I believe that this is two-fold though. First because Imperial squadrons are cheaper, and second because the ISD is a BEAST of a ship, representing the biggest ship currently out to date.
Thankfully the ISD didn't have a Squadron value of 5 as early previews suggested:
http://www.belloflostsouls.net/2015/04/star-wars-armada-wave-2-announced.html
otherwise the disparity would have been even greater.