I think that there are a lot of athletes that partake in a wide range of sports and ectomorphic "swimmer body" would be the prototype for these. I had a friend who was 6ft 4in 194lbs at 42 (personal trainer at a $100/month gym) and really was a super athlete. 3hrs of cardio a day was not uncommon.
People really don't see highly dedicated athletes of this type though. They may go to the gym and are respected or they may work out at home, but they are some of the healthiest people on earth.
That said I think anyone could build impressive size and look really good whatever their metabolism or bonestructure. It may be harder but life is more of a challenge than not.