Yeah a big part of the cost of the house is not the structure or the square feet but the utilities/bathroom/kitchen. You have to bring in sewage, water, electricity, heating (natural gas?). You have to put in an electrical box. You need a furnace of some type installed. You need a sink in the kitchen, toilet, sink in the bathroom, bathtub. I would guess all of those things are 25-30k. From that perspective, yes you could probably add another 100 or 200 sq feet of just space for an extra 10 to 20k.
However, there is something to be said for minimizing your impact on the environment, keeping your heating to a minimum, etc. The other benefit of a tiny house is that it encourages you not to be in it! It becomes more of a crash pad than a lounging zone. It also encourages less stuff accumulation. It also cuts down on the cost of future maintenance.
The only issue I see with this particular situation is that they don't own the land and that will create some complexities. Hopefully they have thought through how they will exit this. Regardless, I think they will be able to sell it in a heartbeat given comparable costs.