Why is the tonnage of general warships smaller than that of mail ships (cruise ships)?

Warships require high speed, generally more than 30 knots, wind and waves resistance and fast steering. To meet these requirements, the energy efficiency ratio should be the highest within 10,000 tons, rather than the bigger the better, regardless of economic benefits. It is based on the combination of war demand and its own technology. Cruise ships should have economic benefits, so they are bigger and can earn more income.