There are Australia and Madagascar, which are either large islands or small continents depending how you define continent. A dingo or fossa could bring down the largest kangaroo or lemur. But the thing to realise is that in both these cases, as with the komodo examples, these are not “natural” situations.
The islands of the komodo chain were once home to elephants which, although small by elephant standards, would have had tough enough skin to withstand dragon attacks as adults.
Australia was once populated by buffalo sized wombats, giant kangaroos and a range of other fauna. The dingo itself only arrived a few thousand years ago, although before that the continent was home to a similar marsupial predator, a jaguar type marsupial, terrestrial crocodiles and giant monitors and pythons. But none of those would have been a threat to the herbivorous giants.
Madagascar was once home to giant birds as well as a lemur very similar in habit to a gorilla, but significantly larger. Neither would have been prey for a fossa.
I don’t know enough about Kodiak to be sure, but I suspect there may have been mammoths on that island before the arrival of humans. After all if deer managed to make it, it seems improbable that mammoths, which dispersed to many more remote islands, were excluded. If there were mammoths then we can scrap that example as well. Kodiak bears are big, but not that big.
In all these cases, as in the case Colibri’s example, we have an artificially impoverished system that hasn’t stabilised since the last big extinction event. If they had time to recover and if people weren’t involved almost certainly the herbivores would outgrow the predators once again.
The only example of a large island where the largest predator really could tackle anything is New Zealand. It’s a bit of an odd case though because there were no mammals. The largest predator was a giant eagle, while the largest herbivore was a flightless bird about twice the size of an ostrich. The eagle could have taken down adult moas simply because flight gave it such a massive advantage, and because the moas were vulnerable simply from having a long fragile neck and small head. A mammal of a similar weight would probably have been effectively immune to eagle attacks.
