They are, and that's what has led to this current generation of seat and restraint. Intamin mentioned in a lawsuit that their previous lap bar and seat design found on rides like Millennium Force was designed for a rider that's around 150 pounds and in the 5'5" height range. Flashforward to today, the new generation of seat and restraint has doubled the rider weight without sacrificing security for smaller guests.
The inherent design challenge with making something "bigboy"-friendly is that once you make the restraints large enough to facilitate larger body dimensions, you start to run into an issue of the restraints not conforming to smaller or younger guests. So they aimed to find a middle ground that alienates as few riders as possible.