[masa]

sono senaka
Chiisaku furuete iru

It doesn't mean that "Your back is actually small" but means "Your back looks small (because of the lack of self-confidence or so)". So I may have to change the translation like this.

Your back looks small and trembling

How do you think?

And I think this expression is poetic, as Misa-chan said. "Senaka ga chiisai" is not a common expression, though we commonly use the expression, "Seneka o marumete" (hunching one's back).