when you have a few litres of headspace, you've got a nice blanket of co2 to sit on there. if you pop the bucket open, it doesnt take long to build it back up and be protected again.
if you pop it open when you've got 12 litres of headroom, it'll take considerably longer, and that leaves it open to all sorts of airborne nasties, particularly before and after fermentation. you're fine to have a peek of your full batches every now and then, but do it with a lot of headroom and you can easily spoil it - trust me, personal experience :p