The obvious third alternative is that somebody wants to preserve the TOS nacelles for the TOS look exclusively, perhaps with the intent of passing them off as "more advanced than the current stock" when the need to do a "In a Mirror, Darkly" style stunt predictably arises.
The bridge size and the idea that the saucer has ten decks plus the bridge (with the brig on Deck 9) both favor a ship whose saucer is more or less similar in diameter to that of the TOS ship. The TOS one supposedly also had about 11 decks, even if it was a lot less voluminous because it didn't bulge out on the dorsal side except at the very middle.
A ship with a 130m-150m'ish saucer would be about 300m long, much like Kirk's. The Shenzhou could be a bit bigger, depending on how we squint. Making her smaller would take some doing, though, with the "Deck 9 within saucer, decks down to 15 exist to take damage" facts at our disposal.
The Discovery lacks a deck count so far, so we don't have good guidelines for our squinting. We can eyeball the five-deck-tall shuttlebay and come up with a ship a tad bigger than Kirk's but with giant nacelles that make her twice as long, but we may be eyeballing it wrong until we do get that deck count. And the MSD from "Butcher's Knife" doesn't help us along much, as its (thankfully) barely glimpsed deck count appears way too low to match the shuttlebay or mushroomroom specs. So, still waiting for relevant dialogue!
Timo Saloniemi