Poor muscle tone?

I am 25 y.o. male, healthy, occasionally exersice, 5'10'', but weight 125 lb. I don't feel any discomfort, I just wonder why I have no muscle mass. I go to fitness club occasionally, ski, run, do swimming. I have well-developed muscles on legs, but my arms, shoulders and chest only have sinew muscles, and seem to have no or very little development. My wrists are basically just bones covered with skin, they are very thin.

I did a research online, but all I could find was abnormal conditions such as dystrophia, which is not what I have. I don't think this is a disease, it's just a certain condition that for some reason keeps me very skinny. I know, there are probably some people reading it and thinking "I wish I'd be you", but still...

It's in your genes.