This is the 265th most frequent Swedish word.
liten
Small.
Here, 'lilla' is the definite singular form of 'liten', modifying the noun 'katten' to describe it as small.
Den lilla katten sover.
The little cat is sleeping.
Here, 'liten' is the indefinite singular form of 'liten', modifying the noun 'fågel' to describe it as small.
Jag såg en liten fågel.
I saw a small bird.
Here, 'liten' is again in the indefinite singular form, modifying 'hund' to describe its size.