The way I look at it you have a choice. You can either have a small umbrella with you at all times, a folding insurance policy, or take along a large one when the weather calls for it.
I've opted for the former. It's easier to maneuver, fits in a pocket or bag and is light to carry.
But what it boasts in portability it lacks in coverage. It's the diameter, I guess. There simply isn't enough nylon to keep all the drops at bay.
I think there's a life lesson here; I'm just a little too soggy now to figure it out.