I think that honesty is your best weapon. If your co-founders are truly intelligent and motivated, you can probably convince them to think bigger. Try to do it in such a way as to show you thoroughly understand the current idea and its merits, which demonstrating its shortcomings. Just be sure to be nice about it; people tend to get angry when you talk about their own ideas that they put so much time and effort into.

Your course of action, I guess, really depends upon how far along you are though. If you're near a complete beta or working release, you might want to just ride it out.