Humans are visual creatures, and with nothing in the image to provide visual scale, there's no "gut feel" for the size of the thing.
So, Will Gater (@willgater) posted the next image: "For a bit of fun I dropped a scaled ISS into that great new @ESA_Rosetta image of Comet #67P: pic.twitter.com/m5EwM5MP9f"
In case that's hard to see, I added a circle:
And then, the comet's size snaps into focus.
Rosetta's not a particularly large comet, either.
So now, statements like "The dinosaurs may have been wiped out by a cometary impact..." make a lot more sense, eh?