It's difficult to assess without seeing the ankle.
If the bump is towards the outer ankle but on top of the foot, it is the extensor digitorum muscle belly. You could have a very developed muscle there.
If it is below the outer ankle on the, it is a fat deposit. Sometimes, it can be bigger on some than others. If it is towards the inner ankle, it could be some sort of tumor...so, you would want to have it examined. The fact that it is on both ankles, it sounds like a congenital formation (you were born with it and it is normal for you makeup).