[dogscoff]

Plenty of mods address the issue you mentoin, namely keeping smaller ship sizes useful throughout the game. Many mods use enhanced speed and to-hit modifiers to do this- smaller ships are made faster and more agile- and another factor is often cost, especially in mods like P&N where you can start filling up those kt with ultra-expensive armours and weapon mounts.

It's a while since I played Devnullmod, but I believe both of those techniques are used, to a certain extent. That's why your AI ships are still using older hull sizes.