First, some words on my fitness history:
I was quite a scrawny kid growing up but in high school I joined the high school's Cross-Country team and started running regularly for part of the year. I found that each year as the season started and I worked on getting back into competition shape I would gain about 10 lbs. and my legs (specifically my thighs) would increase in size. I thought this was unusual, since distance running is usually associated with toning muscle rather than building it, but building muscle is what happened. This may be in part due to the fact that I lived in a mountainous part of the country where some of my training runs would often have elevation gains of one hundred to several hundred feet. We would also incorporate hills into our speed workouts. Particularly for me, I enjoyed running hills and so would seek them out on my own personal runs. This may have contributed to the increase in muscle size. I also lived in a higher-altitude environment (about 4500 - 5000 feet) so the air was thinner. To be more specific: 2005 started Cross-Country, ran summer and fall 2006, spring, tried out lacrosse for a season, wasn't very good. 2006, summer and fall, Cross-Country again 2007, spring, track (1600 m and 3200 m) 2007, summer and fall, Cross-Country again (I sprained my ankle shortly before the season started so I was out of commission for a little while). 2008, spring, track again (1600 m and 3200 m).
I then ran a little bit in spring of 2009 recreationally.
Summer 2009- summer 2011 spent two years in a humid sea-level environment where I did a lot of walking but little running.
I didn't start running again until summer 2013, but didn't get to run very long or very often because I was dealing with shin splints. There was nowhere flat where I lived so every run consisted of at least a 40 foot elevation change.
However, from spring 2012 to spring 2014 I started biking regularly to school, which I lived about a mile away from and had a significant downhill one way and a significant hill the other way.
I believe my thighs and buttox are disproportionately bigger than the rest of my body. For instance, I have to buy relaxed fit jeans that are at least 2 inches too big for my hip size just to fit my butt and thighs comfortably into them. For instance, my thighs measure 27 inches EACH while my hip measures at 36 inches and my waist measures at 35 inches. I do have some stomach fat, but since my legs increase in size while I was in good physical condition and had almost no fat on my body I am sure that most of my leg size is coming from muscle.
Most of this leg volume was developed running Cross-Country in high school but I believe some more leg volume has been developed in my exercises since and possibly even gained some fat since the beginning of 2012 (that's when my face and body started looking less scrawny and I started getting some stomach fat).
I have been afraid to take up any sort of exercise again in fear that it would again increase my leg size and I would need to buy pants way larger that my waist/hip size just to get my upper leg to fit.
First question: how did my legs get so big? Was it any particular exercise I did or am I just a genetic anomaly?
Second question: I now live in a flatter part of the country but I still am planning on biking to school most every day and there is a little bit of a hill. How should I approach this biking to keep my leg muscle size from increasing?
Third question: What kind of exercise can I do with my bike that will allow me to burn fat and build strength/endurance without increasing my leg muscle size?
Fourth question: What kind of exercise can I do IN GENERAL that will allow me to burn fat and build strength/endurance without increasing my leg muscle size?
I am a 24 year old male. I would also prefer exercises that don't take a lot of time (whether in preparation or implementation) since I am in a busy stage of life (aren't we all :)? ).