I don't know what happens in the UK. In
America it seems to be pronounced like Marjie, but here in South
Africa the g in
Margie is the same as the g in
Margaret. This makes more sense as a short form of
Margaret, but I don't like the sound of it at all. I far prefer
Maggie, or even
Meg or
Peggy, though they would be unlikely in today's world.