I agree EVOs can normally be chosen smaller. Its a consequence of them being rather thing and wide, so they have less volume for a given width. Many twin fin boards on the other hand, have very narrow tails and also quite doomed decks and pack a lot of volume in a small effective bottom surface area. That is one reason they ride small. But it is also an effect of the twin fin setup itself whch sort of gives the rider more command over the rail of the board even with comparatively little effort. So you can make a rail driven turn with a wider board than you would otherwise use. The Starboard twin fins from 2009 are still like EVOs rather flat and wide and overall only have marginally narrower tails, so they don't ride as small as some other variants. But on the other hand they are VERY effective relative their volume in light wind.
Quads have some of that "easy to engage the rail" feel from twin fins. But they also have a lot more drive in the turn and grip off the top. Twin fins rely on a phenomenal looseness, you can very very quickly redirect the board and go where you want. Off the top they are easy to control, so you can attack a lot and put them in gnarly places and still come out. On flatter faces they might slide out, also sometimes when you don't intend them to. The Quads more rely on drive. As mentioned, it is still very easy to get them railed up, but there they accelerate much more than twin fins and more than single fins too. You can either use this to drive very vertically up the face or to make a longer turn where the Quads will keep their speed in an amazing way. And in the top turn you can turn as tight as on a twin fin but you will get much more speed with you out of the turn. You can also make the turn higher up the wave and still fins grip. An extra benefit of the Quads is a phenomenal upwind ability. WIth some technique, you can go upwind really well on twin fins too, but the Quads are just in a different league.