Think of the pencil marks a family draws on a door frame. The child keeps rising; the marks stay exactly where they were. Outgrow can be pictured as growing past a line that once fitted you. The coat, the cot, the small garage, the old fear — none of them changed. You went past them. In one use the line belongs to the thing you left behind; in another, the line is simply another grower you moved ahead of.
The source records outgrow simply as out- plus grow. That is the whole picture: growing out past something.
來源把 outgrow 記成 out- 加 grow 兩塊組成。字面就是「長到外面去」，這正好就是它今天的畫面：往上長，越過原本的那條線。
Wiktionary entry for outgrow, etymology_text supplied in this packet: "From out- + grow."
Two branches
長大到原本的東西裝不下
這一組的共同點是：被比下去的那一方是靜止的。衣服、嬰兒床、一間車庫、一個小時候的習慣，它們都停在原地，是主詞自己長過去了。
verb · B2
To grow so big that something no longer fits you, so you can no longer wear it or use it. It is used most often about clothes, shoes, and things made for a small child, such as a cot or a baby seat.
長大到穿不下
長得比對方快
同樣是越過一條線，只是這條線會自己移動：對方也在長。所以重點從「尺寸」換成「速度」。
verb · B2
To grow or increase faster than someone or something else, so that you end up bigger than them or ahead of them in number.
長得比…快