A ほど(~ない)=not as [adj.adv] as A
A より=more than A
ほど~ない is a set phrase.
ex;
She is not as young as you.
彼女はあなたほど若くない。
In an affirmative sentence, we usually use と同じくらい, not ほど.
ex:
彼女はあなたと同じくらい若い。
She is as young as you.
I hope my English makes sense.