The idea that the canonical biggest battleship class* is a "third-rater" because historically only big clunky flagships and failed experiments were bigger strikes me as another place where incompatibility of assumptions between the Honorverse and the historic Age of Sail.
Honorverse ships are, we have reason to think, designed much as real world modern ships are, with lots of computer modeling and extensive, detailed calculation of every aspect of their design, such that their performance parameters are well known, calculated, and very consistent. Age of Sail ships were built according to rules of thumb by craftsmen who were in many cases only semi-literate and certainly not doing extensive calculation in most cases. They were good at what they did, but there would by nature be an element of improvisation and experimentation in constructing any new category of ship, and there was little guarantee that any two ships would be exactly alike.
In the Honorverse, if a category of ship would be impractical because of its great size and being past the point of diminishing returns for mobility, it is simply not constructed. There is no equivalent in their semi-recent history of the Mary Rose or the Vasa, nor should there be. Furthermore, since ships fight with guided missiles and ECM systems rather than with gunpowder cannon at point blank range, single very large ships as "flagships" are a very reckless design concept, asking to be focus-fired into oblivion by massed barrages from the entire enemy fleet.
This largely invalidates a lot of the motives to build the Honorverse equivalent of 100+ gun capital ships in the first place, as these were often seen in real life as flagships or as superheavy combatants to 'anchor' a line of battle consisting of later ships. Both roles are utterly invalid for capital combatants in Weber's overall paradigm. Because, as noted, Weber doesn't really grasp the history of the Age of Sail very well at all, and the points of commonality between the Age of Sail and his setting are nearly all superficial.