Per the TMP TNG, the Ambassador Class entered service around 2325, which would have been roughly 40 years after the Excelsior Class did. So that strikes me as a reasonable time-frame between classes. We do know the Klingons and Romulans were both still frisky at the time the Ambassador was being designed and produced, so it does make sense that she's bigger then the Excelsior to offer a more powerful tactical response to both Threat powers.
I still maintain that the Galaxy Class was big because she was meant to operate for years - if not decades - well beyond any Federation support and as such needed that space to hold enough supplies to be totally self-sufficient for that period of time.
I still maintain that the Galaxy Class was big because she was meant to operate for years - if not decades - well beyond any Federation support and as such needed that space to hold enough supplies to be totally self-sufficient for that period of time.