Historically, the crew size of ships and combat vehicles or aircraft has gone down with advances in technology...
No such trend in spacecraft yet, but then again, starships have precious little to do with today's spacecraft.
The way I look at it, Earth started with crude multi-stage rockets like launch vehicles used in the 1960's. (The Saturn V as an example; observe the small command module and LEM) Then that flagship was replaced with the Space Shuttle.
"Space Seed" suggested an evolution: the larger (but still tiny next to Kirk's Enterprise) DY-100.
Whatever the Valiant was could be a missing link between the advanced DY's and later ships like the Ringship and the NX, both of which are smaller than the Connie.
I'm assuming that the hull designs gradually grew in size as each succeeding generation developed superior abilities for life support, artificial gravity, reliability, etc., to support larger crews. This would also be assumed retroactively from Spock's comments on "crude" space vessels in "Balance of Terror", suggesting that historically older ships have lesser abilities and lesser crews.
Courtesy of Drex Files:
That's before the warp nacelles were added - but one might well speculate that this is what happened in the real world as well, and the surprisingly early launch date of the Valiant is the result of the makers taking an already existing sublight spacecraft and just bolting on the newly invented FTL drive.
This design reminds me of the San Francisco from the Starfleet Museum
. Thanks for sharing.