Welcome to the gastroenterology community! In my opinion, but I am not a doctor, it doesn't sound like the polyp is growing at all. Are you talking about a gallbladder polyp? Ultrasounds aren't get precise which is probably why there is a small change in the measurement.
My aged father had non-cancerous polyps in his stomach that were causing pain and discomfort and he had them surgically removed endoscopically - he's been fine since then.
I would imagine your doc would want to do a biopsy to make sure it's nothing serious.