After YoshiP mentioned it would technically be possible to simply change the size of your house interior when you enter it (considering it is simply an instance), everyone knows all of us would change to a mansion as soon as it happens. No one would stay with a medium or small interior when they can get all the space in the world + 400 item slots (or more, if they go through with their promise to increase the slot limit)

My suggestion is simple, make two separate systems:

Interior Size & Item Slot Limit

Both of these systems would be accessed in a similar way to how we change our interior design, except it would cost gil as YoshiP suggested in the LL. Whether it costs gil to change every time or it's a one-time purchase is not really important to me, personally. The point is that this way, people can make use of the different housing layouts while not feeling restricted by the item limit tied to them, and thus you would see more variation in the houses you can visit when you go around the neighborhoods.

I personally would love this because I bought a medium house, I love it, I don't want a personal mansion, but I do want more slots for it. I understand that the item slot limit is there to make mansions the more desirable option, but if you're going to let every cottage be a mansion inside, what stops you from allowing us to spend our gil on the item slot limit itself?