What is the smallest size island to have a population of elephants in recent times? Actually many islands had populations of dwarf elephants due to island dwarfism. So the question should be what is the smallest island to have a population of regular sized non dwarf proboscideans?
Wrangel Island in the Arctic Ocean north of Siberia has an area of 7,600 square kilometers or 2,900 square miles. The population of Woolly Mammoths surviving there until about 2,000 BC was considered to be dwarf mammoths but have recently been considered to be non dwarf mammoths.
St. Paul's Island, Alaska has an area of 43 sq mi (110 km2). as sea levels rose, St. Paul's Island became separated from the mainland and shrank for thousands of years until it reached it's present size about 6,000 years ago. But mammoths continued to live on the island until about 5,600 years ago, plus or minus 100 years.
Thus it seems possible for herbivores the size of elephants to survive on islands the size of Isla Sorna and Isla Nublar for generations, centuries, or millennia. But one might have to multiply the size of the islands for dinosaur populations depending on how much those dinosaurs exceeded the body mass of elephants. And of course the tropical vegetaton on Isla Sorna and Isla Nublar might offer more nutrition per acre.
So I guess the answer is a big maybe.
There was proably an island in what is now Transylvania, Harteg Island, with a population of dwarf dinosaurs. https://en.wikipedia.org/wiki/Ha%C8%9Beg_Island Since Harteg Island had an area of about 80,000 square kilometers, about the size of Hispaniola, but wasn't large enough for full sized dinosaurs, one might imagine that a significantly larger area would have been required for herds of large dinosaurs.
See also here:https://scifi.stackexchange.com/questions/190130/what-would-be-the-area-size-of-land-required-for-dinosaurs-to-survive-and-flouri2