I will draw a diagram when I get back.
Duct work should be though of as a bucket, think of it as a two gallon and five gallon bucket, if you took ten gallons of whatever material, water and our pour it into either bucket it will only hold five or two gallons, the rest will not fit and run out on the floor, no matter how hard you try. Duct work is the same way, a given size of duct will only hold so much volume of air at a given pressure. .
When a 6 inch duct run is run from one fan to the other as the air pass's from one bulb to the other the heat from the bulb is transferred to the air at increasing temperatures, if each one of those bulbs requires 100 cuft of air per min and the tube will only accommodate 100 crm of air total what happens to the heat?
It is transferred to the room correct because it can not be evacuated by the amount of airflow through the system .
This is why if for a example we have 6 gallons of air and only a one gallon tube(expressed in CFM) this is what happens, if you have 6 , 6 inch runs and they connect to one run that will hold all the volume of the six, air will move freely and evacuate the heat.
This also destress's the fans or air handlers. They operate at a lower static pressure, drawing less amps and the fan motors and capacitors last longer.