Why size does not decide whether the Galaxy is a cruiser

Size alone is a poor way to classify a warship. Cruisers, including exploration-era frigates, could be as large as or larger than contemporary battleships. The County class likewise approached contemporary battleships in physical dimensions during the 1920s and 1930s, although displacement and fighting role remained different. The useful distinction is the work a ship is designed to do, rather than its dimensions or mass by themselves.