Well Liz and Jeff didn't know it, but when they altered reality on the chronivac they changed it so where giants existed. In this reality giants live on an island in the Pacific known as Giant Island. It is an island about the size of New Zealand.
They usually stay at the island, but sometimes they visit other countries. They visit for vacation, or even rarer business. They may be giants but they live in peace with normal sized people.
In this reality Liz was only in America to attend college. However, she had to attend classes in the gym, because it was the only area big enough to hold her.
Most of the rest of the world does not have extra large accommodations for giant people. There is no giant cars in other countries. However it doesn't take them long to get some place because each step for them is like 5 yards. If they want to live in other countries it is difficult because there are few housing areas built for them. In this reality there was one special giant lodge in the state which is where Liz is living now. It is 10 mile drive away, from the college but Liz can walk it in 45 min. Likewise in Giant Island there isn't many accommodations for normal sized people.
So it was very rare that the giants left Giant Island and vice versa.