the handlebar and seatpost bag(s) are both compressible.

I know a lot of people do not like carrying a backpack. but I'm going there anyway.

I use a WingNut Enduro. it can carry a lot. much more than I care to leave on my back for too long. it can also handle a 3L bladder which for the most part is crazy huge.
when it comes to expandable, at the end of the day, grocery shopping carrying capacity, I use the Enduro. it will easily accommodate 1/2 Gal of OJ, a crazy huge hoagie sandwich, a pair of crocs, a towel, candy, blah blah blah. plus it has criss cross bungee which allows you add yet more junk to it.

so the deal here, is that obviously it encourages a person to limit what you are cramming in it, being that you have to put it on your back. for end of the day grocery shopping, its more than adequate to handle dinner and breakfast.

both handlebar and seat bags that I use, when in final configuration are not fully crammed. both are rolled down quite a bit. there is enough room in each to accommodate enough for dinner and breakfast.