What caused the narrator to comment that "the giants of your childhood, whom you encounter years later... are smaller in relation to your growth''?

When the narrator, Gene, went to the old tree, he was expecting the enormous tree from which they'd performed amazing feats. The tree that had seemed so frightening, so large, and so dangerous in his youth, now seemed small and shrunken..... not only because he grown up, but because the tree had gotten old.