Let's say that PMDG747 is constant with XXXX consumption of virtual memory and I know it's hard on both physical and virtual memory, but my system can handle it with no problem. So, if you're going to test PMDG747 with 4 latest "huge" things such as C9 EHAM, Aerosoft EGLL, GAP EDDF and FSDT KORD, the latter it the "winner".
Tried the same test with C9 EHAM, total RAM allocation is 925 MB with MS default EHAM and 980 MB with C9 EHAM. Aerosoft EDDF, takes 1.00 GB comparing to 934 MB at MS default EDDF.
So, C9 EHAM takes about 55 MB and EDDF takes 66 MB, not really different than KORD.
Since I do recall from some test I did a while ago in FS9, that PMDG 747 takes about 600 MB just for itself, the numbers fits quit nicely with your 1.6 GB total allocation figures, but certainly not because of the sceneries and certainly KORD doesn't look any different in RAM occupation, comparing to other two you cited.
If you have different results, I would be interested to know how you measured that KORD takes more RAM than, let's say EHAM or EDDF. Of course, there's only one way to test: all other things being equal, you first measure the total RAM allocation with only default scenery, then you do the same with the scenery active, and see the difference.