The "Great Resizing" was a retcon of the size of most starship classes mentioned in the Honorverse novels.
When Tom Pope and Ken Burnside started to develop the Saganami Island Tactical Simulator for Ad Astra Games, they soon realized that the math did not work under the assumptions David Weber had given them; the dimensions of the spacecraft combined with the given tonnage gave them an absurdly low density, with superdreadnoughts being less dense than cigar smoke.
As a result, Weber resized the relevant starship classes to make them smaller, as seen below:
