Hi.. Can understand your concern.. As per your query if the swelling is on the floor of the mouth and it becomes smaller and larger it can be a salivary duct obstruction which gets larger when saliva flow increases like during eating and other causes which increases saliva flow while it reduces in size when saliva flow is reduced.. If it is located over the gums it can be due to decayed tooth or infection in gums.. But most commonly it is associated with some pain or discomfort at some point of time in life.. Impacted wisdom tooth does not cause swelling in the floor of the mouth and if it would have caused such a long standing swelling it would have been associated with some pain and soreness.. Other thing that can be considered is a Tumor (Benign or malignant) but mostly a tumor does not reduce in its size by itself.. It may also be a cyst (dermoid cyst) or ranula in the floor of the mouth.. I would suggest you that a correct diagnosis can be made only after clinical examination and detailed history.. You should see a dentist and get evaluated as the lumps anywhere in the body should not be ignored.. Hope your query is solved... Thanks and regards..