Over the course of Homo-sapiens, we have evolved to become larger, because of diet & selection.  [Even in the last couple centuries][1].  Today, we vary across geography and cultures, too, of course.

Now it is thousands of years in the future (your call) and people live happily on Earth, but have evolved to be much smaller; still in proportion, just smaller even than our distant ancestors.  We've grown in population, but have stabilized the Earth's issues and become highly sustainable: almost Utopia.

What contributing factors helped make us so much shorter over time - even those staying on an Earth?


  [1]: http://en.wikipedia.org/wiki/Human_height#History_of_human_height