"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.