Well when it comes to engine displacement, you're just talking about cylinder crosscut area times piston movement (so not even the entirety of the cylinder volume, just how much air the pistons displace). That 4.0 inline 6 is a big ol' lump, but are the cylinders really that big if you think about it? Much of it is the crankcase, head, the block around the cylinders themselves, etc.

4 liters is just a bit more than a gallon