Both are different species, if they seems fit to have slightly smaller, but with vast superior firepower than the bigger variant, then it's surely what humans actually wanted. Besides, Infinity is the masterpiece from desperate time, using whatever ONI researched and put them all together. Being too massive will lags the production speed, and that's not what they wants if the remaining covenant strength attack the human world once more.



Like this picture, Orion is also designed to put their utmost firepower against covenant ships, especially cruisers, while Athens is easily mass manufactured to be swarm killers against carriers (along with strike crafts), destroyers and frigates. Such duo is enough to stand a chance from scattering threats. For orbital/ground invasion however, frigates are remain necessary to unload troops into hostile world.



And some ships are improved the material design, and no longer having Halcyon-style platings that to occupying more than half of the room. But Halcyon class is a very good tanks against plasma fires, albeit their weak firepower. Usually to be the spartan carrier used to make a storm entry, or to protect weaker ships to flank enemy fleets.