A lot of people who use NFP do so for religious reasons, and they actually want a larger family. On the other hand, you find a lot of people will say that they use the "rhythm method" or that they "tried nfp" and then they exclaim how it doesn't work because they wound up pregnant. Usually these people never actually learned how to do it correctly, and they aren't truly using NFP at all. If you want to do it successfully, then you have to take a course or read something like TCOYF cover-to-cover, and actually follow all the rules.
I'm Catholic and know lots of Catholic families who truly and carefully use NFP. They all have differing family sizes. The ones with one or two kids (like myself) are not ready for more. The ones with 6 or 8 kids, have *planned* each one of their children, and they had that many because they wanted a big family, not because NFP failed them.