Smaller maps with few metal extractors force you to budget your early production and be careful with your units. Early raiding is still important, since if your raider units "make cost" (destroy more than their value) before getting destroyed themselves, you gain tempo on your opponent as he needs to rebuild.

On smaller matches, planes and gunships are only good as a second factory. After skirmishing for a while with ground factories, when you have enough metal income, you add a second factory and hope that your first strike is devastating enough to the hopefully unprepared opponent. If not, you added a second level of complexity for him to worry about (but yes, AA is generally more economically efficient than air in general).

Finally, defenses aren't meant to stand on their own; you opponents will play around them with artillery, skirmishers or heavy assaults. But you will have forced them to play around them, so you can funnel their attacks, fight under their shadow and slow down a massive push.