There is a context to that quote. According to Plutarch, Philipp was being shown a seemingly untameable horse by a breeder. A young Alexander approached it and saw the horse was afraid of its shadow. He held his hands over its eyes and calmed the beast, then jumped on its back and rode around wildly. When he saw this, Philipp allegedly said these words, although it's not entirely certain how they are to be interpreted, whether he was proud of his son or angry at his own humiliation. This scene is in Oliver Stone's film, but I think the quote itself wasn't.
It's not to be taken as an historical episode, though. Plutarch according to the prooimion to his Alexander biography stated that he was a biographer, not a historian - and it was his main concern to paint a vivid picture of Alexander's character. Plutarch was also a moralist, so it was not his intent to write fully historically accurate accounts, although he sometimes used authentic sources, but to use famous people as moral examples for the readers to take inspiration from or to be appalled by. In the case of Alexander, his whole biography has a strong tragic undercurrent, and this scene is emblematic. The implication is that Alexander from the beginning on was driven by a desire for expansion that eventually brought himself and many other people to misfortune (this aspect is actually more pronounced in Curtius' account). To Alexander, Plutarch argues, there could never be enough, he always had to go further, there always had to be more, and he could never be satiated. So it's highly likely the words were not uttered at all and Plutarch made them up to give the horse episode a stronger moralistic character. This is emphasised by the fact that all the other four ancient accounts on Alexander do talk about his horse (the hero-horse relationship being a very old narrative topos), but do not mention this phrase despite it's obvious interest to the whole Alexander narrative.