Does Gollum even wear the ring in the original Hobbit? Or does he just carry his precious around?
In LotR it’s revealed that he rarely wore it anymore by that time.
Its on such an expedition that the ring "slips" from him, further suggesting the ring is actually not only his size, but a little large.
It's heavily implied in LOTR that the ring is able to change it's size to cause itself to slip from a person's finger, though that's somewhat out of scope and the illustrator may not have read that.