"I am a lightweight."
When the Japanese heard someone said "I'm a lightweight," they assume you are a boxer or a skinny person.
When you can't drink much, you can say
Literally translation is "I'm weak for Sake."
んです form is commonly used here because you use this phrase when you are explaining why you are not drinking.
So, what would you say for the opposite person? In English, you can't say "I'm a heavyweight." But in Japanese, you can use the opposite word. Opposite word of 弱い is 強い(つよい）means strong.
Therefore, if you can drink a lot, you would say
「お酒に強いです。」"I drink a lot."
There is a word for a person who can't drink.
「私は下戸です。」means "I don't drink."
I'm a lightweight. I can't drink much.
I can't drink much. I get drunk easily.
Did you drink up a bottle? You can drink a lot!