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They are all much the same size, but they can also all vary enormously in size. Females will lay eggs on a carcass for many days, until there isn't a single morsel of meat left on it. This means that the eggs laid on day 1 hatch into maggots that get to eat as much as they can stuff in, and then emerge as big fat adults. Eggs that are laid days later will not hatch to the same abundant food supply and as a consequence the resultant flies will be less well nourished and smaller or stunted. Those unlucky enough to be from eggs laid right at the end of the process probably actually run out of food and die before pupating.

Good easy to spot clues to this fly being Calliphora sp are the combination of very sharply bending M vein, metallic blue body and the darkened lower calypter (aka squamae).