Why is my lower body so big compared to my upper?

I'm a boy and I'm 17. Its just getting annoying now. Since I've noticed from the age of about 13 my bums big and my thighs are enormous. I'm a sprinter but looking in the mirror (naked ;D haha) with only shorts on I don't look it. Your ideal sprinter would have a bigger upper body slimming down towards the bottom (slim but very strong and muscular legs). Plus there's an issure with my arms being skinny and its just becoming annoying lol. Even doh I do a lot of upper bodyy exercises there's barely a difference. I'm 6 ft weighing 13 stones and 11lbs and I run the 100 and 200m. O yh my question is why is my lower body clearly larger than my upper and how can I slim down my thighs and get rid of my HUGE *** but still keep its strength? Extra info: my dad is slim but not skinny and fairly muscular for his age (48) and my mums kinda overweight but its unlikely that I got my physical features from her. Sooo answers pls? No joke