That's more criticism against your "intelligent designer", you know.
WHAT increased brain size? I suppose you havent seen this article...
Evolution doesn't strive for something or have some goal in mind (unlike the claims of the ID supporters). Evolution is a description of what happens. The simple observation that traits are inherited, new traits occur, and these traits influence the survival rates of their carriers is the core of evolutionary theory.
In answer to your question, if we lived lives that 'punished' small birth canals more severely (in other words, we eschewed our technology and modern conveniences to help mother/child survival), we'd see an increase in the size of birth canals. Eventually. Under ID, the increase in brain size and the increase in birth canal size would've happened at the same time which is contrary to observation.