A baker's cyst is a a fluid filled pocket behind your knee.This is also called
popliteal cyst.
The excess fluid is usually caused by conditions such as
rheumatoid arthritis or
osteoarthritis that irritate the knee. It may also be caused by an injury.In your case obesity also may be the cause.
symptoms are,
Tightness or stiffness behind the knee.
Swelling behind the knee that may get worse when you stand.
Slight pain behind the knee and into the upper calf. You are most likely to feel this when you bend your knee or straighten it all the way.
sometimes pocket of fluid drain to lower leg tissue due to rupture may cause redness and
severe pain.
Treatments usually pain killer,and have to treat the cause for the bakers cyst,
elastic bandage around your knee,resting to knee,drain the fluid with a needle,shot of steroid medicine,rarely it may needs surgery.
Anyhow dont worry,this is not that serious condition if your taking proper treatment. In your case i suggest you take steps to reduce weight and to stay at healthy weigh.