With a tall equation like the following, using simple ( and ) gives brackets that are too small, and using \left( and \right) gives ones that are ridiculously big:

a+b\left(\underbrace{\sum_{i=1}^{n}f_i(x)}_{h(x)} +g(x)\right )
I need the brackets to stretch in a way that takes into account my equation may not be distributed evenly across vertical space - e.g. the above case is very bottom-heavy.
How to achieve this?
