Thank you for writing Justanswer with your question.
It can indicate a problem that could lead to a pulmonary embolism.
I will explain this too you.
One leg larger than another in a person who is totally ambulatory, can indicate one of 3 problems. These are deep vein thrombosis (DVT), which is a blood clot that has formed in a deep vein in the leg; atherosclerosis with a clot of blood in an artery; and lymphoedema.
The first two conditions are of a more urgent nature than the lymphoedema.
A clot may break loose and travel to the lung or elswhere and could cause a pulmonary embolism, which would require emergency care.
With a DVT, besides the swelling the leg can feel warm and may be redder than the other leg. Usually an ultrasound or sometimes a venogram is done to check if a clot or other damage of the vein is present. Treatment is anticoagulants (blood thinners) which can be given intravenously, by injection or in pill form. Sometimes this is needed for up to three months and sometimes the individual will stay on anticoagulants the rest of their lives. Other recommendations are to elevate the leg, use a heating pad (never longer than 20 min a time), TED or other compression hosiery. Sometimes a filter must be placed in the vein to prevent clots from going to the lungs.
Atherosclerosis with a blood clot in an artery is of more long term risk and could lead to a heart attack, stroke or TIA's, or pulmonary embolism as well. Often both venous or arterial thrombi or blood clot are not noticable until they do break off and cause problems, such as a heart attack or pulmonary embolism. If a thrombus in an artery is large enough to block the artery the leg may feel like it is cold to the individual, feel tingly, or numb. It will appear lighter in color than usual, since the oxygen rich blood is cut off, and if it is a large artery the individual can loose the limb.
Finally lymphoedema is caused by damage or blockage of a part of the lymphatic system. Lymph is clear and drains excess protein and water from the leg and returns it near the heart. If a large lymphatic vessel in the leg is blocked, it will make the leg appear larger than the other as the lymphatic excess protein and water will be trapped in the tissue. This is treated by manual lymph drainage (often by a physical therapist or an occupational therapist), self draninage techniques, compression garmets, elevation, and sometimes pneumatic pump compression.
If you have any indication that a clot has formed and is causing the swelling in one leg to be larger than another, you need to have an investigation done quickly, so that the proper treatment can be started. It is usually safe to at least start taking one aspirin daily to prevent additional clotting. Of course if you have any difficulties with stomach acid or ulcer or any other reason that you can not take aspirin, please do not take it.
I hope this helps.
If you have questions, please reply.