Lion ISO size VS. Snow Leopard
The Developer Tools (760 MB) included on the Snow Leopard DVD are not part of the OS X Lion installer. They are a free separate download in XCode 4.1
A whole whackload of printer drivers (~400 MB) are not included in OS X Lion. Presumably they are downloaded when a new printer is added.
The Snow Leopard DVD includes a Library folder (1.3 GB) with frameworks etc. needed because the DVD is bootable, while the OS X Lion installer creates a bootable partition for installation.
That said, the Essentials.pkg is actually bigger in OS X Lion: 1.35GB vs. 882 MB


Those are the major differences that jumped out at me, I'm sure there are other minor differences I've glossed over. The actual "Installation" package on the Snow Leopard DVD is ~4 GB, the OS X Lion installation DMG is 3.74 GB.