The Co-op at Stadelhofen is open on Sundays. A bit pokey, but certainly larger than the HB Migros.

Off Topic but this talk of Migros reminds me of one of the comedians who came to Funny Laundry in Zurich.
He asked a member of the audience who he worked for. "Migros", came the reply and the comedian dumbfounded said, "What!.....Negros?"