Black platform Mary Jane's size 9 and 1/2 or 42

I just opened the package and tried them on and they were half size too big so I'm reordering them in my size which is apparently not a 42 or 9 and 1/2 in these particular shoes? I'm a 10 normally to go figure LOL. they are black shiny patent leather tight material with a black and white scallop star on the outer portion of the shoe with two studs in the middle. they have a chain with three star dangling across the metal of the toe. they have a strap across the middle of the foot to hold your foot in that fastens with three adjustable holes and flat studs across the top of that strap, six studs around the back sides and a platform base.