Currently small housing is incredibly small compared to the medium size house, so much smaller that many FC's would either wait to afford a medium house, or not buy one all together. If the idea is to have a gil sink make it so that people will actually want to throw their money at it. A small house is the most achievable, however it's the same size as your inn room, which is for 1 person! Also there many many more plots of small land than medium, as the prices slowly go down the medium plots will all be purchased because no one wants to bother with the pathetically small plots. Only leaving the crazy expensive Large plots. A gil sink only works if people want to use it, and right now no one wants to use it, the size of the small plot is one reason, along with the obvious reason of the high prices.

Make it so small housing has a basement, therefore making it more tempting to purchase, thus creating a better gil sink. Or even make it possible to build the basement and an attic as an add on.