Evolution is a very gradual process of adaptation that can take thousands, even millions of generations for any noticeable differences to occur. It doesn't happen in a dozen generations because some species experiences an improvement in its diet. LOL!
All that's happened is that we're now, as a rule, growing to our full potential because we aren't being hampered so much by poor diet. So, we aren't getting taller, it's just that our recent ancestors were shorter than they were supposed to be
