The human body is not completely symmetrical as it might seem to you. In most men, the left testicle is slightly bigger than the right. Checking the testicles thoroughly for the first time often reveals this fact to a young man and the fear it may produce is not uncommon. However, the testicle might be bigger due to an inflammation and swelling, but pain usually accompanies this condition. As for the pain in the upper leg, the location is not precise enough for any assumption or conclusions to be given.
You might want to ask your family doctor about the pain and to exclude more serious conditions that include an enlarged testicle. There is nothing embarrassing in going to the doctor. Your health is worth it.