The way that someone is "built" refers to their body type. If a person has a lot of muscles, you say:
He's athletically built.
If someone's overweight, you can say:
She's rather heavy-built.
And for thin people:
My brother used to be built kind of slenderly until he got married.
Another way to say it is that someone "has a ___ build":
He has a kind of solid build, right?
A person who is "solidly" built is pretty big and strong-looking, but not extremely fat or extremely muscle-bound.