Bra size isn't important in story writing as for a reader it means very little, one man's C cup is another man's D cup. You'd do better comparing their size to familiar object:
"Oranges that grew into grapefruit's stretching her top as they continued to swell into a pair of magnificent water melons."
Or there's always the sports ball option:
"Her small tennis balls inflated soon reaching volleyball size, the seams on her top breaking under the strain as her volleyballs expanded into soccer ball territory..."
Use things that the reader can relate to, bra sizes are too fluid in size and are totally different across country boarders, a UK D cup is substantially smaller than an American D cup.