Even for big projects there are often good reasons for which they don't accept a certain contribution. The project maintainers usually have a broader understanding of the application, so more often than not, they find important flaws or coding style issues in contributed patches which the contributor has to address before the patch can be included.

This is often perceived as bureaucratic and timidity against changes, but usually this is nothing more than a self-defense against subtle bugs, unnecessary complexity and vanishing readability, for which big projects are more vulnerable than small projects.