This probably doesn't need to be said, but here goes: Being big is not the same thing as being great. This applies to most things and all organizations, and certainly applies to all nonprofit organizations. If your primary rationale for support is that you are the largest -- or the oldest -- nonprofit in sector X, then it is time to rethink your value proposition. If your primary goal is to get to Y size, then it is time to find a goal that actually relates to impact rather than appearances.
Corollary: I have noticed that there is an inverse relationship between organizational size and organizational passion. This is not a universal rule, but it does appear with notable frequency.
There's nothing wrong with growing, and there's nothing wrong with large organizations. But size is not the purpose of anything we strive to do. Impact is. The challenge is to endeavor to be great and to grow while doing it -- while not breaking the things that make you great in the process.