When did warships weighing as much as WW II era heavy cruisers start to be referred to as destroyers?
According to the Washington Naval Treaty of 1922, destroyers were limited to 10,000 tons. The Z comes in at 14,000. The Type 055 comes in at the limit of 10,000 tons.
Cost of the Z $7.5 billion
Cost of Type 055 $750 million
So, the question should be is the Z better than ten Type 055s.
After the 80s and the BunkerHill/Aegis class cruisers and the tail end of the Spruance class destroyers is when things started to get all funky.
Cost was the main thing driving designations after that with smaller ships being classed up if too expensive and larger ships classed down to give something of an illusion to the shrinking navy. It all had to do with politics and what people were willing to put their name to on a vote for funding as I understand it.