I noticed this, too. I replaced my MK2 buttons and sticks a couple years back when I got it with HAPP products. The competition sticks feel pretty good, but have a more hollow, light-weight feel to me. And, like you stated, the buttons from HAPP sit a bit taller (so it seems they have more travel), which I don't really like.
It is more evident to me after getting a dedicated MK1 which sits beside the MK2 now and it is OBVIOUS there's a difference in the buttons, both height AND even the color. It COULD be that the buttons on the MK1 are slightly faded, but they look not as bright red as the HAPP buttons. I just measured the diameter of both types, and they are the same. (at least my tape measure says they are) Don't have anything more accurate to measure them with. The original buttons appear a little more concave than the HAPP buttons. Might be why there is an illusion of them being wider.
If you find anything closer, shoot me a line, I'd be interested in buying some.