Did you own the Yuin's? If so, would you say that the housing of the MX980 is bigger than the housing of the PK2? I think the housing of the PK2 is just slightly bigger than the PK1.
The PK2 is as big as I can do on an earbud. I don't think the OK series would stay in my ears. They are just too big.
No, I never owned any of the Yuin line. I'm basing my comments on some owners' reports of premature breakage. I'm not implying the Yuins are poorly made. It's just that portable buds get thrown around, shoved in pockets, dropped, etc., and I think the MX980's can withstand that kind of treatment much moreso than the Yuin's.
As for the size of the MX980 buds, they are a little on the large side, but I've worn them while walking and doing various physical activity and they seem to hold their place nicely. I think the foam pads help keep the buds seated properly. Of course, your ears may vary!