This simile from James Hurst’s “The Scarlet Ibis” is meant to show the vulnerability of the character of Doodle, who was born disabled and not expected to survive past infancy. The first-person narrator describes him this way:
He was born when I was six and was, from the outset, a disappointment. He seemed all head, with a tiny body which was red and shriveled like an old man’s. Everybody thought he was going to die—
The fact that they are able to name him at all, when he was not expected to live past infancy, is a victory of sorts. The name they choose, William Armstrong, sounds like a name for someone who is robust and healthy, which is certainly not the case with Doodle. Perhaps it is wishful thinking, or an acknowledgment of his will to survive, that inspired the name. The narrator, however, sets the tone for the rest of the story when he says:
Such a name sounds good only on a tombstone.
Over the course of the story, Doodle makes progress and exceeds expectations, learning first to crawl, then talk, then walk. However, the narrator’s ambitions for him, as an older brother, eventually lead to a tragedy that is heartbreakingly told in a detached, matter-of-fact style that makes this a truly sad story.
Doodle was such a tiny and sickly baby that he was not expected to live. His parents waited three months before naming him, and when they decided on a name, they gave him a big one. The significance of the quote--
"They named him William Armstrong which was like tying a big tail on a small kite"
--is that the name was both long and symbolically powerful for such a "shriveled," weak little baby. The name "Armstrong" denotes a person with a strong arm--a mighty person. Doodle was hardly that. The simile that follows compares the name with a soaring kite, but it is a kite that will never be able to fly: Like Doodle's tiny body and large head, the small kite and big tail are a mismatched pairing whose flight will be a short one.