Length, hull volume and T foils !
I've sailed boats at 14, 15, 16, 17, 18 and 20 feet.
My Stealth F16 with T foils is VERY stable, never had any issues with pitchpoling; Stuffed it in a few times but not (yet) pitched it in. Similar to boats much longer.
I think Hobie 14 and 16's are by far the worst!