Ice Age Giants

Animals tend to grow bigger in colder climates as they follow Bergmann's Rule, that being a the larger you are the less surface to volume there is; this means the bigger you are the warmer you are as you can heat yourself faster and more efficiently. So in cold environments, large is optimum. Clearly food sources play into this as well, African animals tend to be large as well, but food source is the primary driver there.

But in the ice age, big was good.

That means Trolls shall Survive!