Hi segfault,
Congratulations on your new mattress 
As you know I think you made a great quality/value choice and it’s good to hear that you are sleeping so well on your mattress.
As you can see at the end of post #116 here … there was a small percentage of a few of their production runs that had an issue with expanding to their full size or in some cases that were taking longer to reach their full size. While the reason this was happening has been resolved … it’s possible that yours was one of these and if you have measured your mattress correctly and it still hasn’t reached it’s normal dimensions in another week or so (see post #2 here) then if you give them a call they will be happy to help you.
Phoenix