I cycle long distances at least 1-2 times a week. Why are my thighs getting bigger?

Last summer, I trained for a half-marathon. During this time, I noticed the space between my upper thighs got a little bit bigger. Not a lot, but still able to see it from afar. I cannot afford to buy proper running trainers at the moment, so I had to give up running after the half-marathon in October. Now that I am on my summer holidays, I feel like the hole between my thighs is closing rapidly, so much so that I can only see it when I bend. I cannot see if from afar through a mirror. I cycle long distances from between 6-13 miles at least once a week and I walk for at least 2 miles everyday. Is it because cycling does not burn as much fat as running? I cycle on very steep hills so after cycling, the edge of my buttocks and my whole thighs hurt. I notice my inner thigh also hurts the most when I am cycling uphill.

Do I perhaps need to reduce my food intake? I also notice my thighs have gotten bigger. I am not sure if it is from fat or muscle. I have also started swimming last week, once every week. I am a black 20 year old female by the way. My cousin, who is a doctor, tells me that black people get muscles much faster than any other race.