Mokele

Given sufficient time, yes.

Think of it in terms of energy budgets. When a snake is young, it dumps any energy not used for basic life functions into growth, and thus grows very fast. As it gets older, it's growth rate slows, and it becomes able to reproduce. At this point, the snake has to split energy into 3 things: basic life functions, growth, and reproduction. Because an animal obviously can't short itself on the first (or it dies), it's mostly a choice between growth and reproduction.

Imagine you have 100 WC rattlesnakes, and you feed them all the same and keep them in identical conditions, measuring and weighing them every week. After they've all grown up, you'll have a curve for each snake, which starts out with a steep slope (fast growth), which gradually flattens out, but each snake's curve will be different. Naturally smaller snakes (even if only a foot shorter than normal), will have a curve that isn't as steep at first, and flattens out sooner and at a smaller size.

Now, imagine you dumped those babies on an island, and they got stunted. Let's say, hypothetically, they were all now just 4 feet long at sexual maturity. For the babies who, under normal conditions, would have gotten quite large, they're still on that steep growth curve, and any calories from the mice they eat will be diverted much more heavily towards growth than reproduction. But for naturally small snakes, they may already be at the point where their growth is slowing, and so would spend more of their energy on reproduction.

This leads to a fairly simple system - naturally smaller snakes would reproduce more, leading to a smaller next generation. That generation would face the same issue, and the smallest of those would reproduce more. Finally, after many generations, the natural size of the snakes would be so reduced that they're no longer stunted, and if taken into captivity and fed freely, they'd stay small.

The key issue is how long this takes, and when you find the population. If only a few generations have gone by, captive ones would probably reach full, normal size. If it's been a few thousand years, it's probably genetic. If the island is an uplifted coral atoll or a volcano that only became large enough to be habitable a few hundred years ago, they're probably just stunted. If, however, the 'island' was colonized by snakes via a land bridge during the last ice age (which ended about 10,000 years ago), they've had plenty of time to adapt and become true genetic dwarfs.

Hope this helps,
Mokele