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.
Half of this song is excuses and the other half is somebody standing at the door. One voice swears they are five minutes away, that they left the house at two, that whatever went wrong was not their fault. The other keeps checking the same thing: you said you were already outside, and you still are not here.
What gives it a sting is the second half of the chorus. You are always late exactly when I feel small and need you most. Lasso sings that as chiquito and Sofía Reyes answers with solita, so the same complaint comes back from both directions. Those two words are also a neat piece of Spanish to notice, since the ending shifts depending on who is speaking.