I'm still new to all this - one leg of a V with with my wife as the hinge and her bf as the other leg. And while I am potentially open to another partner, I am mono at the moment. I have met the bf once. So - I'm really only having to deal with my wife having another partner - and as a mono at heart, that is easier said than done - but managing. However, I do see her having to deal with two relationships - which would be just a simple double - but she also has to deal with the dynamics of the poly relationships - both of our reactions to her having another partner and potentially the (non-sexual) relationship between me and her bf if we progress that far. So - not merely the double of two relationships but add another factor to deal with the dynamics.
I heard a couple raising two kids say it this way - the second child didn't just double the effort - it at least triples it because not only do you have to deal with 2 kids - you have to deal with the interaction between the two of them - and that can be no small thing! Thus the logic for "exponential".