What would cause a short fantasy race, like dwarves, hobbits and/or goblins, to maintain their smaller size outside of the environment that produced it? I know that the exact height of these races fluctuates between different examples, but what would cause the average height in a setting to stay roughly the same, or not change at all, if, say, a population of dwarves moved out from their underground home and emigrated somewhere else? How could you justify them remaining small without the environmental factors that provoked that development or any real genetic exchange with the dwarves that remained?
If you don't need them to evolve short, just stay short, then they only need an advanced enough society that they rarely die for lack of stature. The only way evolution is going to make them taller is if tall folk are breeding faster. This can happen if tall folk are seen as more attractive, or short folk die too young to have kids.
Assuming that short is seen as perfectly sexy, and short folk aren't getting killed while hunting or in battle, you should be fine.
Me Da was a tall one for a dwarf. Poor guy took din get married till he was 632. And me Ma was even older. Romance is hard for a tall dwarf. No one wants a skinny mate. They want a spouse whose nice and squat and wide too. Not like you humans with your long limbs, so skinny and breakable.