After living on a too-big 44' houseboat for a few years I figured out I didn't need most of that space and I'd rather have the freedom to go more places. 99% of my belowdecks time is spent at a desk, in a comfortable reading chair, in the galley or in bed. On a 20-footer those happen to all be the same place, but move a few cushions around and it works just as well. I could wish the head & shower were a little more convenient, but if they were I'd probably be wishing I could still float in a foot of water or pull it to a new cruising ground behind a car.
If I was going to have someone else aboard for more than the usual week or two at a time I'd probably look for a different (though not necessarily larger) arrangement that would let us get a bulkhead between us when needed.