I think that is the wrong direction to go in. It is not the size of the boat but the motion of the ocean. It probably has to do with the stimulation she is getting or she is picking up on your anxiety from it. I would count your blessings that she doesn't really care and try to enhance the sex you are having. That is a very common issue.
Life is a cruel taskmaster and sex and intimacy get postponed.
Decide for yourself that you want this to happen. You need a cooperative partner. Talk about how you would like things to be and not how they are wrong. If you stumble and it doesn't work that well, by all means, see a therapist. It is always a pleasure to have you as our guest.