To use the dutch as an example.
150 years ago the dutch had one of the shortest heights of all the european nations, now however they are the tallest. In 150 years that is quite impressive seen as that can also be viewed as about 6 generations. This is normaly seen as too short a time period for evolution.
Human evolution is normaly not looked at over such a short time period, the romans 2000 years ago (80 generations) are considered to be geneticaly very similar to modern europeans.
Most complex phenotypes are interactions between genetics and enviroment, if we can rule out the change in genetics and look at the dutch diet and wealth we can see something intresting.
150 years ago the dutch had been trading with the east and the wealthy of the dutch classes became even richer. When the dutch became a liberal democracy in the mid 19century they also in a very short time became the tall nation of europe. This may have been to the increase in the average wages of the lower classes or the advances in healthcare.
It is not due to a weaking of gravity. For gravity to be weaker there must the earth must essentialy lose mass. This tends not to happen on anything other than cosmological time scales.
