My own mod reflected similar concepts. There are larger hull types available at start, but they are 'inefficient'...ie, they have maintenance penalties etc. Researching Ship Construction (and some other prerequisites) allows faster, cheaper, and better defense mod hulls. Also, the sizes of each hull increase slightly as the game progresses (ie, a CA might be 550kt at start, but up to 650 as the tech increases).
I simply like the concept of having a 'balanced' fleet available from the start. I prefer to reward players for using a base fleet of CAs and CLs with various small fry for escorts and using the Capital ships only once real wars are declared...they are simply too expensive to keep around at all times.
These changes allowed (and required) some real tweaks to the overall combat dynamics. I had Armor mounts that only the larger ships could use that made it very difficult for many smaller ship weapons to hurt Capital Ships. So, you have to use CAs+ to really mount weapons capable of taking on BBs and DNs. There are specialized weapons (mostly Torps) that can cause enough damage and are able to be carried by small ships. These weapons are inaccurate enough to be nearly useless against smaller ships, so trade-offs must be made. Capital Ship mounts are also extremely inaccurate vs small ships, so smaller batteries must be included or else escorts assigned to keep them safe from roving hordes of torpedo ships.
All in all, it feels like a more 'traditional' naval model. I really liked the effects, but it was like herding cats trying to make the AI use it effectively. In the end, we gave up on the AI and switched it a MP-only type of mod.
One of these days, I will go back and re-create that mod for the latest Versions of Gold etc.
Sorry for the ramble.
Talenn