There is some truth to that, though. If you put a cup of same-temp water in each and unplugged them both and came back in an hour, the cup of water in the cabinet would be warmer.
While 50% is 50% and 99.5F is 99.5F regardless of volume, several factors DO impact incubation conditions.
Air flow. With a large amount of fresh air exchange, more water will be taken up to maintain the same humidity. Warm, moist air will exit the incubator and cool, drier air will enter...