It was also more expensive. DW and HttT are like two DLC expansions in price, but not necessarily in content.Mmmmm....
I wouldn't exactly compare 1.06 + SoI to HttT or DW. Those had super drastic changes like an updated map, CBs, nations, decisions, ect. ect. on a much larger scale than that of CK2.
But also take into consideration that HttT and DW (probably) took a significantly longer time to develop than either 1.06 or SoI, so I guess it evens out/is proportional.