I'd suggest checking out Mission Workshop backpacks. Their designs are really smart, well built, and (most importantly) stylish. I bike commute daily with the Rambler as my every day backpack. It's pretty heavy (3.7 lbs), but it has carbon fiber stays and a frame sheet, super durable materials, is fully waterproof, and expands from 22L to 44L. I've put awkward loads like a 20lb box of cat litter in it and it felt fine.
In every day use, I have it in 22L mode and it's super thin and svelte. By unzipping the wraparound zipper, it accordions out to 44L to carry groceries or whatever else you need.