expression, i-adjective
talkative; loquacious
Describes a person who talks a lot, often with a nuance of being too chatty or indiscreet. Used as an i-adjective phrase.
彼は口が多いから、秘密を話さないほうがいい。
He's talkative, so it's better not to tell him secrets.
口が多い人と一緒
Being with a talkative person is tiring.
expression
having many mouths to feed
Refers to having a large family or many dependents to support. Used as a set expression, not as an i-adjective.
口が多いので、毎月の食費が大変だ。
With so many mouths to feed, the monthly food expenses are tough.