You’re right that there was an energy trade-off, but it was being able to run faster and longer that was more important than strength for our ancestors, who still had quite small brains (the brain of an Australopithecus is only 35% the size of a human).
Brain size developed later, probably in a feedback loop with our diet - as we began to eat more meat our brains got bigger, which made us better hunters. And hominids actually got bigger and stronger as their brains grew.