From Halopedia, the Halo wiki
Even though both UNSC and Covenant fleets possess warships of such classifications, the tactical philosophies of the two navies are different. The Covenant, while possessing many different cruiser classifications, use assault carriers as the powerhouses of their fleets. The UNSC, on the other hand, use cruisers as their heaviest warships, designed to engage and destroy enemy warships that may otherwise threaten a fleet's carrier force. Cruisers served as the backbone of human fleets. However, it should be noted that Covenant cruisers were considerably longer than their UNSC counterparts.