Think of it as "diminishing returns". The first tiny bit of fin added is hugely beneficial. Then the next bit a little less, and next bit even less. a longer fin is always better (more heat transfer) than a shorter fin, but each bit of extra length you add does a little bit less than the piece before it. Eventually you've essentially maxed out, so extra length added does practically nothing extra for heat transfer, so that lowers the efficiency. A long fin, that gets longer, you add very little heat transfer, but a lot of extra length, so this is inefficient.

@@BrianBernardEngineering thanks!! That's a great way to put it!!