It is "standard" operating procedure, in hot air ducting, to decrease duct size the further it gets from the heat source. The same amount of air in the 4" duct gets put into the 3" duct. This increases the velocity and helps move the heated air along. I just hope the is a direct path out of the bathroom and back to the return duct. Also hope that air is returned through the cabin and not through the basement. The return air is going to suck the air with the least resistance. Hopefully it is not the air from the drain 'scuppers'. That air might be 40°, or more, cooler than cabin air, and the basement air might be 20° or more. The warmer the return air the more efficient the furnace is at heating.