The word liten is an irregular adjective, meaning "little" or "small." So, here are the forms of liten: en liten hund (a small dog), ett litet hus ( a small house), den lilla hund (the small dog), det lilla huset (the small house), den lille pojken (the small boy), or de små hundarna (the small dogs). So, små is just the plural form of liten.
Why the difference between den lilla hund and den lille pojken? Could you also say den lille hunden / den lilla pojke? Or what am I missing?
You can use the -e ending for adjectives describing males in the definite. It's entirely optional, and a remnant of a gender system Swedish used to have, a long time ago.
Then what is smått? I thought that was the neutral form of små but that doesn't make sense if små is only used in the plural form.