[Say there's one fluid ounce of headspace per bottle, so roughly 50 fl oz (1500 mL) total, versus maybe 500 mL (one pint) of headspace in a keg.
I don't think the difference is even that dramatic. Assuming the headspace is formed by removing the bottling wand, then a 3/8" diameter wand in a 9" tall 12oz bottle would leave 1/2 oz of headspace per bottle. Also, 1 pint in a keg is 0.44" of vertical space - many homebrewers are leaving more space than that. So headspace is probably close to equal or even greater in a keg.