Small ships were all the could be build with the remaining infrastructure of the RSE?
My guess is it is all they can man post-Empire, or they prize versatility now. More hulls means more ships can be more places at the same time. Smaller hulls also means each loss sacrifices fewer people, if they care about that, and the loss of a smaller hull means easier maintenance post battle on surviving hulls. It's the difference between writing off a hull and having to rebuild it completely versus just making a new one and that would be a huge difference with a D'Deridex, but a small ship like the ones we saw could probably be built new in a matter of weeks instead of months or years.
That being said, I will be disappointed if we never see D'Deridex and Valdore in later seasons. There should be Romulan governments formed around single D'Deridex and whole remnant fleets. Some governments would be captains or admirals turned warlord, others would be lucky local governments turned independent. I really want the show runners to read Jack Campbell's
The Lost Stars, because it basically is about the breakup of the Romulan Star Empire without any Star Trek in it. Or, just get Jack Campbell on as a producer.
The sheer number of ships was ridiculous. However, I am not surprised as most Hollywood writers know squat about the military. In the real world, the US Navy uses one or two ships to launch attacks against land based targets. They may be part of a carrier task group, which has at least eight ships and a few more depending on the mission.
A more realistic scenario would be a small number of Romulan ships, some to hit the planet from orbit. They may have a troopship with the fleet for this specific mission, to land troops and to clear the settlement of survivors. The Federation would likewise have a small fleet, a squadron, which is smaller than a fleet. And, unlike what was seen in the episode, the Starfleet would stay in orbit until everything is settled.
The fleet sizes fit the peak of the Dominion War with fleets of a hundred and combined fleets in the thousands, and unlike the US military post-WWII they wouldn't have to worry about funding like we would if they wanted to maintain that peak capability. Even personnel issues shouldn't be that hard to get around if they go for more enlisted heavy crews over officer heavy crews. As in train on the job for specific jobs, as apposed to training each officer for every job all at the academy.
Just for comparison, if the US military has 3.3% GDP but taken from the entire Earth's GDP it would be 4.4 times larger. If it scales linearly then we would have:
48 super carriers
96 cruisers
13 Zumwalt
294 Arleigh Burke class destroyers
Multiply that by 150 (member worlds)
7200 carrier
14400 cruiser
1950 Zum
44100 Burke
The only times I know of where there was a large number of ships was when there was an invasion. The Romulans weren't invading, they were raiding.
I would say the best comparison is "The Die is Cast." In both cases we have a fleet heading to a world to sterilize its surface, or vaporize its surface down to its outer core (which eliminates any need to land troops) but this time it is with a fleet of far smaller ships, implying far less firepower per platform. 30 ships did the job in "The Die is Cast" and now we have 200 and change, but in the former half the ships had 6 times more volume than a Galaxy class, and just eyeballing it I would say the average Romulan ship in the new fleet could pack 150 in just the Galaxy class saucer section if placed in 5x5 grids stacked 6 deep.
The fleet ran un-cloaked, and with long range sensors Starfleet would know what it was heading into so it makes sense it would send a fleet of similar size rather than the squadron originally promised.
I believe the only unrealistic part of this is how Oh didn't ignore the various opponents and just fire on the planet from the start. She should have been enough of a fanatic to have done that.
Don't forget how Hollywood treats Space Battles like the "Age of Sail" where every battle is within "Visual Range" because of the "Rule of Cool".
I believe that has less to do with being cool than a failure of imagination. There are several anime out there with beyond visual range fleet fights which manage to be visually impressive and exciting. Heroic Age, one of the Space Battleship Yamato movies, and Legend of Galactic Heroes all do it. Also Babylon 5 has at least one if not two beyond visual range fight scenes which are fantastic if brief.
