Depends on the breeds compared. Less than half a standard sized bird easily. But some standard breed pullets go 7-12 lbs... At which point a normal bantam is pest sized.
One of my sizzle cockerals loves a Delaware pullet who is half his age, twice his size and kind natured toward the silly little fellow. As the weeks pass she grows, he appears to shrink. When she's full grown he's gonna be about knee high.