Yeah, I remember noticing the same thing about my bella, back in the day. It eventually did grow a big root system, but it seems very poky about it. And some cuttings, yeah, just seem to hang in there without roots forever. Although my rootless cuttings seem to die eventually, unlike yours, which are supported by magic?

I don't know what advice to give you, other than to maybe give it some seaweed fertilizer or willow water. I don't think you should worry. Just don't underwater the plant. Mine struggled a bit when I moved it to my Hoya mix, because I had a harder time keeping it as hydrated as it liked.

Some hoya species naturally create smaller root systems than others. Because yours grows fine, I would not worry either.

Your experience underlines a rule some hoya growers including me follow: it is to root cuttings directly into the mix adult plants will be growing in. Because some root systems are small, AND because most hoyas like being root-bound, it's good to remember that when you stick a cutting into a pot, it may very well prefer to live in that pot for a long time. Transferring it from the rooting medium into the growing medium seems to create unnecessary work for you and stress for the plant.

If you do repot them though, choose a pot that is only slightly larger than the root ball. I don't know how many cuttings you had, but a 6-inch pot was probably too big for them. People generally start even the biggest cuttings (bigger than H. bella) in 3.25-inch pots at most.