There’s a fairly long tradition of showing Mary Marvel’s breasts as bigger than Mary Batson. In the early days of the comic both were shown as children and so that wasn’t done, but as time went on Mary Batson was often shown as a young woman and since Billy transformed into a muscular hunk when he said his magic word, Mary Marvel was likewise shown a bit on the overdeveloped side.
The image above is by Shade, and shows Mary Marvel with huge breasts and getting a cleavage-porthole like Power Girl, thanks to the the magic lightning.
Adam Hughes has parodied this on several occasions – here’s one.
Alex Miranda has produced a number of cheesecake images featuring the character as incredibly busty – there are a bunch here.
Anyway, I don’t think Mary Marvel exists in the current DC Comics continuity, but hopefully she’ll back. And hopefully the Mary Batson character will be a beanpole who grows into a big-breasted Mary Marvel whenever she says the magic word!