Density...
The IAS logic says that if you take an airplane of given weight and wing area and, say, double its true air speed at a given density, the aerodynamic forces, both drag and lift, quadruple (and lift no longer equals weight). If you decrease density of air, both drag and lift decrease proportional to density. Thus, if you were to, say, double the speed of aircraft and simultaneously decrease the air density four times (typical to the cruise altitudes!), the drag and lift would be both unchanged.
This logic holds true at small Mach numbers, but breaks down on approaching the speed of sound.
Now, if you climb to cruise at constant IAS, the density of air, in that example, decreasing 4 times and the true airspeed only doubling, while the cross-section of engine inlet is unchanged - so the mass of air scooped up per unit time is decreased by half.