The heavy cruiser Olympia alongside the mid-level craft Orville.

"The Orville's only midsize. You should see the heavy cruisers." Alara Kitan[source]
Heavy cruiser refers to the largest vessels in the Planetary Union fleet.

Ed Mercer's teachers at Union Point predicted that he would be captain of a heavy cruiser before age 40. His first command, however, would be the mid-level USS Orville. Reviewing the credentials of chief medical officer Claire Finn, Mercer noted that she could be posted on a heavy cruiser, however, Dr. Finn chose to join the Orville as she believed she was better needed by him.

Admiral Ozawa commanded a Leviathan-class heavy cruiser, USS Olympia, which dwarfed the Orville.

When Sargas 4 native Lysella expressed her awe at the size of the Orville, Alara Kitan revealed to her that it was only medium sized and that even larger heavy cruisers existed.