Im not sure if maybe if your born with micrognathia. to get a large jaw can exercise some certain muscle for it?

No benefit: Micrognathia represents either genetic factors or compression of the jaw before birth by a hyper flexed head. In the later, the jaw will tend to grow out within the first months after birth (as seen is some with pierre robin sequence). Exercises or manipulation has no benefit. Surgical extension of the jaw is possible but should be attempted only by those with extensive experience in the procedure.

Not really: Surgery is unfortunately the only solution.

No: Micrognathia (actual definition micro=small gnathia=jaw) is rarely associated with environmental fetal issues and in the vast majority genetic factors, both result in being present at birth. There are no known exercises or any functional way of changing this until the proper stage of growth allows surgical intervention. Surgery is the only way of correcting micrognathia.