Why are some peoples muscles skinny and like tight with muscle but dont have big muscles?

like me im kinda regualar like im sorta stockey im 13 never had really skinny arms and stuff but i look at people with big arms and they never show off but these kids that are lanky looking have 6 packs where u see there bone and they show off there muscles and it looks more like they have lil muscles but when they flex its like a golf ball and u can tell they natrually have a 6 pack couse there all bones but there the ones that show off and i look at them like tool bags my question is just like why dont there arms get bigger just skinny and tight if u get what im saying