Because the larger one is a baby Brown-headed Cowbird.
Cowbirds are not very good parents. Instead of raising their own young, they sneak into the nest of another bird, lay an egg, and leave.
Cowbirds don’t make good siblings either. When the young cowbird hatches, it immediately destroys any other chicks or eggs in the nest, so it can get all of its foster parents’ attention for itself.
Other birds, for whatever reason, have never gotten wise to this trick. The foster parents happily raise the baby cowbird as if it’s their own. When the young cowbird gets bigger than them, they just think they have an especially healthy chick.
It seems some cowbirds have successfully hijacked a Chipping Sparrow nest near That Yard!