Why mixing kanji and hiragana? Learning japanese and having a really hard time understanding this

Hi!

I'm just starting to learn japanese and I'm really confused. I know why hiragana and katakana and kanji is used, but I don't understand why we mix kanji and hiragana when the kanji character already means the word. For example:

Kanji for small is – 小 which is pronounced ちい (hiragana for sound chii)

But then we have to write 小さい which would sound like chiisai

So I understand why we would use 小 to replace ちい because it makes the whole word shorter. But I don't understand if the kanji 小 already means small, why do we have the second part?

I'm wondering if it's because of pronunciation?

Thank you for any help!