Dude sounds like its just your body type, personally I'm on the heavier frame side of things, just needed to think of sugar and dairy to put on weight, the weight gain from hiking could just be a bulk muscle gain in your major muscle groups, in the glutes, quads etc. Take up some cardio, I find puting the old stationary bike in front of the idiot box with the console and putting in around 2 hours a day is a cinch, Seriously once you get into the swing of things the excess just melts off.
And if your finding such simple gains from just hiking tinker around with some light weights you may be shocked at how quickly your body responds. Oh and I've also learnt using scales ain't the best way of judging your health, go for a body mass index, caliper test etc. Check your blood pressure, blood sugar etc, and stay hydrated.
Never contemplated anything as drastic as lipo especially at such a young age.