Kerbal Space Program

Some of the new aero parts by porkjet (at least, that's who the cfg blames) are duplicates of each other scaled up or down. When scaling, the mass and lift were scaled, which is appropriate. The drag coefficient was also scaled, which is probably a mistake since the drag equation already takes size into account.

This makes e.g. two elevon4 have the same mass, lift, but half the drag of one elevon3, so your plane will fly more efficiently if it's cobbled together from more, smaller parts. More importantly (because much more mass is involved), a pair of Mk2 short tanks have less drag than the equivalent single Mk2 long tank.

I found this by inspection, and it was confirmed by in-game experiment here:
http://forum.kerbalspaceprogram.com/threads/99072-A-funny-thing-happened-to-the-aero-parts?p=1520526

The fix is to muck with the drag coefficients in the .cfg files.