the real pros can straighten me out here if i am wrong but here is something i always think and i could very well be wrong...so here goes
lets take your house and a water hose...you house us putting out 40 pounds pressure, you have a outside tap with a y on it..youhave a 50 foot garden hose on one side of the y and the other side shut off. you open the tap and lests say for fn you are getting 3 gpm out the hose....now you open the other side of the y with another 50 foot hose on it,,,because you have now given that 40 pounds pressure some where else to go so you have eased the head pressure in the first hose and added head pressur in the second hose,,but you have likley reduced the velocity at which the water is moving so in fact it is moving slower but it will not be half or what it was so lets say now it could be moving 2 gp in each hose so now you have 4 gpm...i think a larger heat exchanger may do this as well...you are capable of moving say 6 gpm through an exchanger that has maybe 8 tubes but your pump is capable of 10..so you are pushing with the force of ten but due to friction you can only get 6 through the exchanger...add now, an exchanger with 16 tubes you are adding length to the pipiing and you think it would add double the ehad but becasue you are lowering the velocity of the water the resistance is much lower so you get more gpm through the larger exchanger
the pros can step in and tell me if i am on the right track or not i hope i am typing what i am thinking...lol