I'm no expert, but I could see the need to keep it smaller being a structural deal. Given the placement of the carry handle, if the pocket was larger and went all the way to the back, if it was open and you tried picking a fully loaded bag up by the handle it could cause significant stress on the corners of the opening. By having that extra piece of material in there you sacrifice the storage space for greater stability and potentially fewer warranty claims.
This stinks to a small degree, but if the above is true it makes sense from a business perspective. Personally, I would have liked the extra storage, but all we were really ever told was that it would fit a wallet, phone, and keys. That's what's going into mine. Also, I'm sure there will be folks out there that will come up with a mod or two to open it up (and void the warranty).