Purchased 2 of the ex-ex-large for my 2 Rottweilers cot beds, and they love them. The cot seemed to pull their hair out when their blankets moved and werent supporting enough. They arrived each in a small box with a vacuum packed mattress and cover. I let them sit for 2 days to expand and breathe, and the cover was very easy to put on. Im not sure why the ex-large is 4 thick and the ex-ex-large is only 3.5. Would have been nice to have the other 12 for my large dogs. Time will tell on their durability.