For the past few years I have been taking the bus home, and as part of the trip I usually hop off and get a few items at the grocery store, then hop back on for the rest of the trip home. The amount of stuff I get is limited by what I can carry, which is what I can fit in my reusable shopping bag and in the ‘other’ bag I had brought my lunch in.
Now as I am bike riding home, the amount I carry is going to be limited by what I can store in my bike bag and its’ panniers.
I love the bag I have
http://www.rei.com/product/710404 . The top expands, the pannier bags lock onto the bottom of the rack, and the bag itself locks onto the rack just behind the seat. It also has handles to carry it around and an extra bottle storage compartment. Pretty nifty.
The question: how much can one fit into a bike bag? Here is the answer…
This is it. The Topeak, stuffed to the gills. At home, after a stop in Sunflower Market.

The top. This is a small, second storage area. Great for keeping bike map, has a pocket for keys, good for pant-leg straps, etc.

The main compartment. Almost all the groceries I bought fit in here, with the exception of bread. It’s actually not much as I would typically get, as I was unsure how much the bag would hold, and probably 2/3rds of what I could fit into a reusable shopping bag. Maybe it would be better to put the groceries in the pannier bags (at least the non-perishables, the center portion of the bag is insulated)?

The right pannier.

Had to move stuff around. Jeans I wore on the ride in this morning (too cold for me for shorts), shirt, online trip plans for a bus trip I had to do the next day, etc.
Left pannier.

Windbreaker/bright yellow “please don’t hit me” jacket from this morning, extra clothes and work clothes, and the bread I couldn’t fit into the main compartment.
All in all, you can fit a decent amount of stuff into a bike bag. I think if I organized it differently I could have fit more in. I will look forward to practicing more.