Now we have got home, both beds appear to be rather too small for the dogs for which they were purchased! I conclude that my dogs are actually larger when they are inside my house. At present, the nice new bed I bought for Rosie is quite empty, and the big squishy bed I bought for Brythen is occupied by a small and enormously smug Bungle cat. Both dogs are upstairs on their old beds quaking at the sheer horror of being expected to cope with new beds.
Edited: New theory,dogs contract in size overnight, and then expand again during the day: this morning she came straight down and plopped herself into the bed that she protested was unfeasibly tiny yesterday evening. And there is a clear margin of space all around her!