In reply to BenP: I've seen them at Decathlon in Nottingham. About the size of a bucket! Also a funky chalkbag that snaps shut using 2 semicircular springloaded bits of plastic.
In reply to TN:
It has just come to me why they want large chalk bags, to keep both hands warm on cold days.
Another idea may be that they have weights in them for training with or for an extended boulder their sandwiches and drink in.