When I ran cross country I was 6'0 and 140 lbs. I cut back my mileage and started lifting weights. I am now 165 lbs and people STILL call me skinny! When I am considered not skinny? I don't think my frame can support much bigger muscles. I might be able to pack on an extra 5-10 lbs of muscle if I try really hard, but I'm almost at the end of the road.