We are getting taller because taller people are breeding more successfully, either more often or more offspring are surviving, or more tall people survive to breeding age. People don't get genetically taller from their diet or anything else they do, other than surgery. They are either taller or not, and then pass these traits on. So taller people have been getting lucky more and therefore have more offspring and the next generation is made up of more tall people, who again breed more successfully, etc, etc. Evolution doesn't happen during ones lifetime, that's not how it works.Maybe I'm picking nits here but we're getting taller largely due to better nutrition, not some evolutionary advantage to being tall.
Also, getting to have children is not necessarily about "out doing the competition" in terms of speed and/or power. Certainly not for modern humans. If that was the case, I'd venture that most of us would not have any kids!