Super Monkey is right. They are really tiny. Microscopic, in fact. Look at the size of the entire metropolis compared to a normal-sized human being.
In the comics, when Kandorians escape, their size always seems to be tweaked "upwards", however, so that normal human beings can see them. But it's ridiculous, of course. Even at five or six inches tall, a city of such people would be FAR bigger than the Bottle City.
There's some sort of enlarging gas so the artist doesn't draw them using a tiny dot of ink (that would still be oversized).
They are smaller than small bugs.
Why can't they be let out to do good deeds? Because the comic book world already has Superman and The Atom. The territory is covered. Then there's the question of just how strong a microscopic Kryptonian can be....
Given the scale of strength demonstrated by Superman, a microscopic Kryptonian would still be hugely formidable... heck maybe even moreso in some ways since they'd be so hard to detect. And then there's those brains filled with advanced technology which become super under our yellow sun. Let them out for a few minutes and their enhanced brain power plows through possible solutions that would take years otherwise. They could potentially solve their size problem on their own, without Superman having to be their savior.
Of course, it's not as cool as a bottled city...