I know I've seen this somewhere on this forum before. I was stumped for ages, but Roraborealis solved it.
Basically, the smaller triangle has a slightly shallower gradient than the bigger one, so much so that we don't even notice it. Switching the two makes the area of the whole triangle bigger, because the big side pokes out slightly, rather than poking in.
Why did the vector cross the road? It wanted to be normal.