your parents, and your parents should understand you if you are really and truly 孝 them. Your parents know very well whether you 孝 them or not no mattter you live in a large house or in a tiny apartment. Don't worry about it too much. Get a big house once you think that you are ready. Otherwise, don't buy a house. I believe that your parents would love to see your happy face rather than a worrying face al the time with a big house. It is someting wrong with your thinking of 孝.