It seems it's got something to do with the cost of licensing; the optimum seems to be a 7CD-set, I was told sets with fewer CDs cost more per CD and so do sets with more than 7 CDs. Also, licensing stuff from more than one rights owner for one set seems to be very costly.
Another factor making the larger sets more attractive is the cost of the packaging and booklets.
Obviously, yes! But still, what's the bottom line between 5000 Pres-sets sold within a few years and 2000 larger sets sold within twice as many years? (Storage room would be another consideration to take into account, I assume? Even if they only print a few hundred at the time, those need to be stored.)
They have to print in multiple thousands to be cost effective - they probably print enough for the entire run. The point I was making is a box for 7 cds costs the same as a box for 4 cds and this greatly impacts the per disc cost.