Right now if during refit ship is bulgeg ship loose 10 % of her speed and get free 500 tons of displacement.
Issue is that added displacement is nominal value 500 tons so even small cruiser of 4000 tons get 500 tons as 50000 tons battleship.
I would suggest that free tonnage getting by bulding the ship is function of displacement.
I think it would add more realistic bulges and hope not difficult to implement.
Interestingly, according to the manual, something along the lines of this suggestion should already be the case - it says that bulges should increase displacement by 2%, up to a maximum increase of 500 tons, which should mean that you'd need to bulge a ~25,000t ship (exact value dependent upon how the game would round) to get 500 tons additional displacement. As such, this might qualify as a bug report rather than as a suggestion.