"A big sofa is a good sofa."
Translation:Duża sofa to dobra sofa.
Such a construction, used more for wordplay (Dobry wróg to martwy wróg = A good enemy is a dead enemy), and not for defining (Sofa to mebel = A sofa is a piece of furniture), customarily takes only the "X to Y" construction.
"Duża sofa jest dobrą sofą" would be correct grammatically, but it would sound kinda weird to a Polish ear. "Sofa jest meblem" would sounds perfectly okay.