You'd need an analysis of the house, the heating system including the duct sizes to determine if changing the duct size would help. The amount of heat you can carry in a certain sized duct at a certain air temperature with a specific fan speed can be determined. Then, the house analysis determines how much heat is actually needed and if the source, air velocity, and duct sizes are adequate. WAGs can get expensive. What often happened during construction, or building settlement, is that some of the seams may have come apart on the duct(s), and you are putting air into the wall cavity. If it is an interior wall, it might not be as bad as if it were an outside wall. Air likes to go on the path with the least resistance, so since the ducts to the first floor are usually shorter, they get more air. But, if your ducts are sized properly, those might end up smaller, while the second floor's ducts are larger so the relative resistance is the same (assuming equal areas and load). You can change the relative resistance by putting dampers in, but if there a lots of leaks, and the sizes, both to the downstairs and upstairs aren't adequate for the required flow to distribute the required heat, it won't help much. You might be able to increase the fan speed, but there are limits. Too high and things get noisey, turbulance and rumblings, and the air temp decreases because the air passes the heat exchanger faster, cooling things off and that could create other problems with longevity, too. Plus, at faster air speeds, like a summer breeze, you feel cooler. It could also be that the return ducts, regardless of where they are, are too small, starving the whole system for airflow. Remember, with the supply and returns, it is sort of a closed system...you can't efficiently fill the system to capacity if it is starved for air on the return.