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.