Cage sizing??

Is my rabbit's cage too small? For context, this is my mom's rabbit she got as a surprise gift on her birthday back in July, so technically this honestly isn't my rabbit, I own a dog. It took about a week(?) after the rabbit came home for his cage to come, and the person who delivered it let us know something about if a rabbit can have at least 3 hops in a cage then it's not too small? So someone fact check me on that.

Before his cage came he was pretty chill, he explored the house for the first few days but after he got comfortable he'd just lay in one spot and chill. He could sit in one spot FOREVER playing with one of his toys, or lay in one spot on the floor just watching tv (since he free roams downstairs and we have the living room tv on 24/7). Obviously he'd have zoomies here and there but for the most part he wouldn't really do much, so we took him to the vet to see if anything was wrong and the vet let us know he was completely healthy, just a really lazy bunny. Sometimes he didn't have to be doing anything to keep him occupied, he'd just lay in one area for literally no reason.

He's a free roam rabbit, he can free roam everywhere but upstairs since my dog is up there behind a baby gate and they do not get along. The only time he gets put up is when it's time to go to bed, and he understands that time cause even before his cage got here when all the lights would go out he'd cuddle up in a corner and go to sleep. He also goes up when all of us go out somewhere like to the store, to eat etc. because he can and will jump over the dog's baby gate. Which even then he's put up rarely because always at least one person wants to stay home. So with all of this taken into account..is his cage too small, even for a lazy non energetic rabbit? I'd understand if he was an energetic, rowdy rabbit that needed space to run and jump but even when in the cage he just lays in one spot and chills. I've told my mom about possibly getting a bigger cage just in case, but she doesn't seem too convinced and I'm only 15 with most of my expenses going to my dog, so I'd have to save up money for my dog and save up money to buy the rabbit a new cage. So sorry for this long question, I wanted to add as many details as possible and even though it's not my pet I still want the best for him.