The 18 capsizes easier because it is more powerful. It isn't less stable, it just has a taller mast and more sail area. Besides, capsizing isn't nessisarly bad, in fact it can be a lot of fun. As far as getaways passing H18s, it happens all the time. Speed on a H18 is more sensitive to tuning than it is on a getaway. A well sailed H18 will go 14 knots when a getaway can only muster 12 knots. A poorly tuned H18 will only be faster than a poorly tuned Tiger.
D-PN Ratings for the two boats:
Hobie 18 (Std/Magnum) 71.4
H16 76
Hobie Getaway 83.3
https://cdn.ussailing.org/wp-content/up ... lasses.pdf