That's the common myth of the Mormon family I had growing up.
Of course, as a practical reality, when you have a large family you are trying to divide up resources with, you tend to buy (avoid) things that you would probably never buy if it were just you and your spouse and maybe another kid running around carelessly.
Also, it's emphasized in pretty much every Christian religion (especially with the LDS) the importance of frugality, and not getting caught up in the worldly things.