22clipper - I'm guessing from your name that you're talking about a Robbie with the governor on. In that case I think the primary impact of the carb heat is on the mixture rather than the air pressure, and it's the governor that's making the changes to the MAP.
As you say the warm air is less dense (not necessarily lower pressure) so the mixture is richer, less of the fuel is burnt, the power output reduces and the rpm drops. The governor then opens the throttle to compensate and that is the MAP rise you're seeing.