In my gift bag for buying VIP tickets to the Arlingtonians for a Clean Environment Hallowgreen 30th anniversary event, I found this little guy. It's called a ChicoBag (named after the city in California) and it's changed my standards for reusable bags.
Most of my reusable bags (like the canvas ACE bag seen next to the ChicoBag) are sturdy but bulky. Since I walk to the grocery store, I have to cram my bags into my pockets, which only works well if I'm wearing cargo shorts or baggy jeans. And forget about keeping one with me in case of unexpected bag needs.
But the ChicoBag is not only small enough to fit into any pocket, it's light enough that you don't even notice it. And it expands to hold just as many groceries as the sturdier ACE bag.
The containing pouch is sewn right into the side of the bag, so once you've unloaded your groceries, you can fold the bag right back inside. Oh, and they're only $5.
I suppose my only concern is whether the light bag will survive dozens of trips to Harris Teeter. I'll let you know how it holds up!