I'm no physicist, but I took that bit to mean that the difference in energy involved in lifting someone is so much smaller than the difference in energy required to convert a 135-pound woman into a 10-pound cat without leaving 125 pounds of squishy McGonagall-bits by the wayside that it's far more plausible for it to be coming from some non-obvious source.

Which seemed reasonable enough to me at the time. But looking back at it, I'm no longer sure it really holds water. Harry's family at that point is dealing with a process with almost totally unknown properties; if you can get energy out of the Source of Magic to levitate people with, it's not too much of a stretch to imagine that you could also move mass into it and then back out. We don't e.g. see the floorboards creaking under McGonagall when she lifts stuff with her mind, so there's no particular reason in context to assume that the interaction has to involve local-ish forces.