sandbox has been heavily implied. Having said that, size restrictions just offer you challenge. We know that Sorna at least is around the size of the biggest available in PC, Nublar I imagine isn't far behind. Tacano and Pena both seem smaller than Matanceras but I imagine there's a good reason linked to the story/ And who's to say the story even requires a park on those two islands? yeah they have 5 star ratings but it might be judged differently, maybe it focuses on the hybrid project (I believe that was suggested as being locked until island 3) and breeding in general
Having said that from what I've seen and judging on the size of the buildings, Matanceras doesn't actually look that small. Clearing trees means you could easily fit the innovation centre, a number of shops etc and a power station for them over by the monorail. And that starting exhibit is massive, we've already seen there's ample space for 3 to 4 herbivore species in there with around 3-5 dinos per species. Truth be told, it's only really the carnivores you want separate exhibits for so while you won't get all 40 dinos on that island (unless you're cruel and have them lined up like battery hens) you'll get more than enough dinos in there to satisfy that 5 star rating
In the video I saw, the paddock that is pre-made in the demo fills a massive chunk of the island. If I can only have like 6 paddocks on an island, it just isn't enough. especially since they're blocking most of the island with an invisible wall.