I would recommend that you see your physician as soon as possible because this might be something as serious as lymphoma. You might require a tonsillectomy for further evaluation.
The most concerning cause for enlargement of one tonsil compared to the other tonsil is lymphoma. This is often associated with fevers, chills, weight loss, and night sweats. Sometimes enlarged lymph nodes are present in other areas of the body. I would recommend that you see your primary care physician for an evaluation. He or she might refer you to an ENT who might want to remove the tonsils for diagnosis.