The Topic, and What About It (は〜が)

由紀さんは目が大きい doesn’t put 大きい directly on 由紀さん, it’s specifically her eyes that are big, not her overall size. は names the whole person as the topic; が narrows in on the one feature the sentence is actually commenting on.

得意 and 苦手, like 好き and 上手 before them, always take が on the specific skill or activity, even though は sets the person as the overall topic. This same double-marker shape is what makes ほしい (Xがほしい) and わかる (Xがわかる) work the way you learned them, が landing on whatever’s specifically wanted or understood, は on the person feeling or knowing it.

Even a question word like どこ can take が in this slot, どこが安いですか asking which one, out of the supermarkets already understood as the topic, has the specific quality of being cheap.