On a whim, I picked up a three cube set.
The largest one is one third of the interior size of my travel bag. (Which fits UNDER the seat, no overhead worries for me. So I pack very light, based on the thought that a long trip is really a series of short trips, thus what works for five days works for five weeks.)
My next trip will be in the mountains of Colorado in spring (snow) and NYC for a trade show. And my mode of transport is my feet.
The cubes are working out great in prepack: largest is cold weather, including a parka in a stuff sack, yes, in the cube; medium is the tradeshow/NYC nights; and the small is tech gear (cords, spare power, earbuds, iPad and iPhone, and some comfort gear like neck pillow and Vapur collapsible water bottle.
The cubes corralled things well enough that my crossbody bag, one spare pair of shoes, and toiletries, added to the cubes, all fit with, get this, room to spare.
My outer bag is a redoxx extra small aviator duffel. I see purses larger than this.
I do think having what I think of as removable pockets allows me to get what I need in a smaller space.
With my backpack, which still might go on this trip as the redoxx has no cross body strap/shoulder strap, I use two cubes.