I agree with not having a lot of wiggle room.
But thats not what having a 5 x 8 is for.
It's basically for fast and light trips where you will not be screwed if it starts to rain.
One of the lightest rain jackets i've found is the Marmot Essence, its 6-7 ounces and costs $150.
The golite can be bought for 58.99 shipped, and doubles as an emergency shelter, plus a pack and leg cover.

