Hmmm . . . Medically speaking, when you say leg, are you talking about your calf or your thigh? I assume you didn't injure yourself since you didn't mention it but we have to ask the obvious.
If you've been on a long road trip, stuck in a car or bus, or on a trans-Pacific or trans-Atlantic flight, we worry about blood clots developing which could lead to swelling, typically in the calf. The problem with these clots is that they can wander off into the lung, like in Serena Williams case.
Sometimes, medicines can cause swelling but usually we'd expect it equally in both. Same with heart failure and pulmonary hypertension.
If you're describing your thigh, it is possible, though less common, to have a clot up that high. There might be issues with the muscle or bone, although you didn't mention a specific mass or pain. Perhaps you're attempting to heal a stress fracture.
With all that said, we all tend to favor one side or the other. The favored side is often stronger & bigger. Perhaps, it's always been that way but you only noticed it recently?
I hate to say it, because I sound like a broken record, but the best advice I can offer is to check in w/your family physician.
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