No need for you to put on your hard hat.
HVAC source duct is reduced in size as it passes the several registers it feeds because the flow is reduced at each register or branch. There is no need for the duct to be the same size from beginning to end because at the end there is little flow, just enough for the last register.
A flue pipe in a fire tube boiler delivers all the flow originating at the fire box to the smoke box, and therefore is a constant size from beginning to end.
Rex Beistle
NMRA RMR
Estes Valley Division