I dont know what this is???

Ok I was loping my horse when I got done I noticed one knee was bigger than the other, I felt and I just felt bone no bump or anything just one knee looks bigger got any idea's on what it could be he doesnt limp or seem affected by it???

Check again for any liquid build-up an/or heat(warmer than the other leg). Has one always been bigger? Definitely keep an eye on it and try trotting your horse out in front of someone who is good at spotting lameness. Have them watch the trot from front, back, and as you circle in each direction.