Using 'have a big mouth' suggests someone may break your trust by talking carelessly.

Trust Implications

It's often used when discussing matters that should be kept confidential or private.

Confidential Matters

Considered informal; the phrase isn't used in professional or formal circumstances.

Reflects not just volume but also a boastful or self-promoting behavior.

Implies someone likes to be the focus of attention, often by being loud or showy.

While seeking attention, the person might get negative reactions instead of admiration.