Spyone,
Yeah, I also wish tone of voice/inflection could be put into posts. My tone was one of suprise. I didn't think the Neubula was that big. I'm not anry or anything more like" Wow, I didn't think that thing was so big!"
My source for the volume of the neubla was the ICON DS9 book. Unfortunately, for me. Other sources, such as the Starship Spotter and CODA Starships book that I have looked at since give a somewhat larger beam for the Nebula, making it about twice as large as it is in the DS9 RPG book. That makes it about half the size of the Galaxy class by Volume. In RPG terms (for ICON/SPACEDOCK and CODA) the Nebula is a Size 7 ship while a Galaxy is a size 8.
Now internal volume, if you are assuming that they both use the same components (BTW another sign that they would be built at the same time), would be a bit closer.
Most sources I have looked at give the comissioning date for the Galaxy and Nebula classes as 2357. NCC regrestries and dedication plaques in Trek aren't very consistient anyway. Else we'd be talking about the Starship-class Enterprise, and worrying about the the USS Constellation (NCC-1017) being considerably older than the Constitution (NCC-1700). It also makes sense tht Starfleet would constinue producing the current class of vessels (in this case Ambassador) until they completed whatever contracts they had to fufill, and until the replaement class was fully testest and deployed. It might also means that some shipyards that could beuild the Amdassador class weren't able to build Nebula's or Galaxy class ships. We also have several instances of ships flying around with mutiple regriesties or the same name being used on mutiple ships in a manner than doesn't make much sense.There are just way to many inconsistiences with hull regestries and dedication plaques to go by them.
Pretty much every thing that I have looked at on the Neubulas indicate that they were designed at the same time as the Galaxy class in order to be a smaller/less expensive companion ship much in the same relationship that the Miranda-class had with the Constitution-class (down to the similarity of componenets and replaceable section).
The appoximately 30 year gap between the introduction of the Ambassador and Galaxy classes seems to be about right. It compares well with the 40 year gap between the Constituion and Excelsior classes, the 40 year gap between the Excelsior and Ambassador classes.
Now the 15 year gap between the comissioning of the Galaxy and Sovereign classes is interestersting. If this is due to a falure on the part f the Galaxyt design reamins to be seen. IMO, the Soveregin was designed to be more combat capable that the Galaxy-class, and desgined to deal with the new threats to the Federation-especially the Borg.