And if I just say "You become thinner, mother becomes sadder", why it is not right?
Subconsciously I understand that it is not right, but cannot explain to myself, why.
Well, i think it's on the emphasis of the sentence.
In your case, the sentence would mean "kamu menjadi lebih kurus, ibu menjadi lebih sedih".
So we put the comparison words in the beginning of the phrase.