Chiquito literally means 'little' or 'small'. It's a diminutive form that can be used to describe size, but also to express affection or, in this case, a feeling of vulnerability.
Lasso sings "Siempre que me siento chiquito" (Whenever I feel small), using the word not to describe his physical size, but his emotional state of feeling insignificant and helpless when his partner isn't there. It's a beautifully poetic way to express a complex feeling.
This song is a super fun duet between Venezuelan artist Lasso and Sofia Reyes about one of the most relatable relationship problems ever: one person is always late. He insists, "Estoy a cinco minutos" (I'm five minutes away) and makes the classic excuse, "Te juro que me fui de mi casa a las dos" (I swear I left my house at two). She's not buying it, singing back, "Dices que ya estás afuera / Pero nunca llegas" (You say you're already outside / But you never arrive).
But it's about more than just bad timekeeping. The chorus gets to the heart of the problem when they sing about needing each other most. He feels let down "Siempre que me siento chiquito, y te necesito más" (Whenever I feel small, and I need you more), and she feels the same, singing "Siempre que me siento solita, y te necesito más" (Whenever I feel lonely, and I need you more). It's a song about how being unreliable can feel like you're not really there for the people who count on you.