This has to do with the overcrowding in R4 and S1. In S1, everyone from 80-120 goes to do the same billero quest cos it gives the maximum xp.
AT lvl 110 and above, you get quests for C3 and S2, S3 S4.
However hunting here is not economical at all. It takes several shots for me to knock out a S2 Muddian (Bow archer lvl 111, using stunshot). Compared to that, Foman/Rontu/Wimos in S1 can be killed easier. But the wz dropped by both is nearly the same..just around 2k!!
The drop rate in S2 is also very poor -infact other than R3 no other place has a good drop rate. So net result is you lose money if you do quest there.
So everyone is forced to goto S1 where you can get a large mob and thereby earn more per mob.
S2 and higher maps are always empty, people only go for bossing.
If you increase the wz dropped by higher monsters proportionally, it at least balances out the odds and makes it viable to hunt here.
Tougher monsters should drop more wz compared to weaker ones, it's only common sense!!
In my opinion, S2 creeps should drop at least 5-8k and S3 S4 niff creeps around 10-12k.