The pieces of furniture DO NOT expand your Mog Safe space. I thought they did, but all they do is give you 1 extra space in the "storage" section of the Mog House. Note that the 1 "storage" space makes up for the 1 space that the chest/armor box/bed takes up in the Mog Safe. The "storage" system is merely a way to decorate your Mog House as much as you like without clogging up your Mog Safe with furniture.
In case you don't understand (if you do, just ignore this example), here's my Mog House:
Mog Safe: Simple Bed, Ancient Blood, Flower Stand, Ceramic Flower Pot, Earthen Flower Pot, Maple Table, Stationary Set, Armor Box, Yellow Jar. Total=9 furnishings (yes, my house is kinda sparse, but I love it anyway) Therefore, my "storage" has nine slots to make up for the 9 slots lost by decorating (furniture has to be in the Mog Safe in order to place it).
Personally, I like the storage system because I always have stuff to auction off (from stealing, looting, and clothcraft), so I put the items that are waiting for auctions in storage. That way, I don't have to rummage through the Safe trying to remember what I need to sell and what I need to keep. It's a great way for me to sort out my many goods (I love being a thief!).
In short, don't waste your money on armor boxes and chests thinking that they will give you extra space, unless you really like the look of extra furniture and have some money to burn. Hope that helps. Supposedly, there is a quest that will expand your Mog Safe, but I don't know what it is or if it is just a rumor.