I take it to mean that they're composed well: there's a balanced and sensible spacing/padding, the lines are a good weight, they are easy on the eye, there's a consistent look and feel between all of them etc etc.
For instance, I could mangle some SVG icons and while you could scale them to an infinite size.. you wouldn't want to use them ;)
These look great BTW!
They look great indeed.
That said, some of the icons reminded me of this article:
What does "quality" mean for SVG icons though? I could choose the resolution to be whatever I want.