Large Costco pizza; tiny folding bike - challenge accepted.

8 slices of cheese pizza in a Demano messenger bag. Sharing space with a Slik tripod, box of salad greens, big bag of croutons, Topeak Morph G pump and various tools and a water bottle. Attached to a 2013 Brompton S6L-X. It was a tinsy bit messy.
Large Pizza - Tiny Bike by 1nterceptor, on Flickr

RANDALL'S ISLAND, NYC by 1nterceptor, on Flickr