Why isn't 大した using 大's Kun’yomi

[plantron]

Okurigana : Kun'yomi and On'yomi

Hello! I have a question about Okurigana. For exemple: 大きさ is an okurigana because it has a Kanji followed by a hiragana, so it means that I should read as “おおきさ”(Kun’yomi). However my question begins here, the word “大した” the “大” is followed by hiragana as well but instead of reading as Kun’yomi “おお” we read as On’yomi “たい”. Why tho? Thanks in advance