I am sorry that your question was unanswered for so long.
A lymphangiography is the next step of the investigations if the MRI and colored doppler study for the veins are normal. Your GP / primary care physician can get this done. The lymphatic blockage can cause this swelling. The primary lymphedema needs surgical treatment.
Please feel free for your follow up questions.
I agree, that MRI can be done to look for some causes like lipoma or lipofibroma which can give localized swelling. If the swelling of calf is generalized. lymphangiography would be more informative. A tendon of soft tissue hypertrophy on one side is very unusual phenomenon to cause swelling. This would be associated with calf weakness.
It is pleasure and privilege assisting you.