Ripped means that you have so little fat that your muscles show through with a lot of definition. You can be ripped without being very muscular. 

Thank you Uly! "Мускулистый" in Russian doesn't necessarily mean "muscular" ("накачанный"), it can also mean "in good physical form, wiry" ("жилистый").

Perfect!

By the way, we also use the word shredded, which means exactly the same as ripped.

Thank you!

Поджарый, сухощавый