Wrong: he had infinite power to do anything he wants so instead of doubling the food supply in the universe (duh!) he halves the population? Come on!

Or he could modify the reproduction rate of sentient species so the problem doesn't come back in 1-5 generations

Or he could alter his own brain so as to no longer care about overpopulation. For that, though, he doesn’t need the reality gem, he just needs the whiskey stone.

Exact power of Infinity Gauntlet is not defined in the movie, may be he couldn’t double whatever is required.