You don't get any gil back for relocating, but if the cost of relocating is more than 30% of the cost of your current plot, the relocation cost is waived. If it's less than 30% than the relocation is discounted by that much.

Personally I would find a large way too big for my private house. I like to decorate my personal house in a way that makes sense for one person living in it, which means I don't want to put in like 5 beds, or a bunch of separate tables. And it is very hard to fill up a large house in that style. My medium house works perfectly as a private residence, though, and I can fill my FC's large house with all the ideas that work for lots of people.

That said, I definitely agree with Vidu that you shouldn't feel guilty about "stealing" a large house. If anyone is upset that they didn't get the house they want, they should be mad at SE, not the player that got it.