I am overweight but still fit in a size small??? ?

I so recently found out im a wee bit over weight by like 4lbs but regardless of that i still fit in size small clothing? how come? And no im not squeezing into a small trying to feel good about myself :p im 5'1 and am anywhere from 129-135lbs (changes a lot) anyway i don't mean to sound offense to anyone or racist or anything but im brazilian and peruvian so stereotypically i have a bigger bottom and a bigger chest than most ppl my age (im 15)... could that be some of the weight? and i barely have any stomach fat too! so am i just a larger frame?