One shouldn't put too much faith in the DS9 Tech Manual stats, which were amusingly copy-pasted to conflicting effect (Bajoran ships have Jem'Hadar weapons, ship mass is proportional to length rather than volume, etc.).
On screen, we only have one datapoint on Nebula crew, if we accept the idea that the Hera of "Interface" with her 800 crew indeed represented that class. But the class identity is not mentioned in the episode. Excelsior crew is likewise only "half defined" - we have 1250 aboard the Grissom of that class in "Field of Fire", but the class identity is not part of the dialogue.
However, if we want to go with the DS9 TM figures, we can argue that the older ships need bigger wartime crews in proportion because they can't be automated quite so extensively, or are more difficult to repair after getting holed by the Jem'Hadar. Peacetime crews might be completely different again.
Timo Saloniemi