It is normal for contrast to decrease slightly as prints get bigger. There seem two principle reasons:
1) Stray light: The intensity of stray light leaking from the enlarger and the intensity of light reflected from the paper to the surroundings and back to the paper is constant and doesn’t change with print size. The increased exposure time gives this stray light more time to act and causes highlight fogging - decreasing the exposure to compensate for the fog causes the shadow density to fall - in either case the contrast is reduced. Of course, if you open the lens to compensate then the stray light/image light ratio stays the same, but you then end up with 2).
2) Increased flare: If the lens is opened up a stop or two to keep the exposure time constant then the lens flare will increase.
The standard progression of print sizes are one stop apart. If you go from 8x10 to 11x14, keeping the crop on the long dimension, then the exposure change is exactly one stop. Ditto 4x5 to 5x7 and 5x7 to 8x10. In each case the linear ratio is 1:1.4, or 1:square root of 2. Obviously 4x5 -> 8x10, 8x10 -> 16x20, and 5x7 -> 11x14 are all two stop changes, and 4x5 -> 11x14 and 5x7 -> 16x20 are three stops.