With increasing experience we learn to sense from moment to moment which speed and bank angle are most appropriate for any given situation. Where a beginner may have challenge enough simply maintaining constant attitude in uniform circles, a seasoned pilot might intentionally vary speed and attitude, perhaps more than once each time around in reaction to a thermal’s changing behavior.
If a thermal is small or weak, such an effort could be necessary to avoid falling out of it. Remember that lift is typically weaker near a thermal’s edge. If your aircraft sink rate is greater than the average lift throughout your circle you might eventually descend through the bottom of the bubble while it continues on up without you, but by aggressively working to stay in or near the core you may be able to ride most thermals all the way to the top!