Use 'say a mouthful' when agreeing with a strong point someone made.

Substantial Points

It's often used to show casual but firm agreement in informal discussions.

Casual Agreement

The phrase isn't about speaking a lot, but about the weight of the words.

Use it to gently indicate someone's talking a lot, possibly more than needed.

The idiom can carry a humorous undertone, implying a friendly tease.

It's fitting when someone gives a long or detailed explanation.