They aren't gigantic but my ortlieb back rollers surprised me this weekend by fitting my bass trombone cup mute (10x10x14"), bass straight mute, tenor cup (9x9x12") and straight mute, sweatshirt/hat/gloves, and books.

As for groceries, I'm only shopping for one, and I never end up filling both of them.

I bring them into the store, put them in the cart, load the cart, reload the cart at checkout and wheel over to a quiet place to load up my panniers so as not to take up time/space at the register.