Magdzia would have been a much better choice.
Nonetheless, it is used by Poles and who are you to argue that? Huh? :)
I am not arguing. You are.
I'm stating facts, so try your own medicine and chill out yourself.
Magdzia would have been as good as Madga as diminutives for Magdalena. They would be very rarely used for Malgorzata.
So let's check the Great Book of First Names (Słownik Encyklopedyczny. Wielka księga imion, Wydawnictwo Europa)Magda:
ZDROBNIENIA (diminutives): Macha, Mada, Madzia
, Magda, Magdalenka, Magdusia, Magduszka, Magdzia, Lena. INNE FORMY: Madlena, Magda, Maria, Marlena.
ZDROBNIENIA (diminutives): Gocha, Gosia, Gośka, Macha, Machna, Mana, Małgorzatka, Małgosia, Małgoś, Małgośka, Masza, Maszka. INNE FORMY: Margareta, Margot, Maneta, Greta.