Better and Bigger

Better adjective - (comparative of `good') superior to another (of the same class or set or kind) in excellence or quality or desirability or suitability; more highly skilled than another.

Bigger and better are semantically related. in greater topic. In some cases you can use "Bigger" instead an adjective "Better", when it comes to topics like larger, excelling.

Bigger adjective - Large or big relative to something else.

Better and bigger are semantically related. You can use "Better" instead an adjective "Bigger".