User blog comment:Jimeee/PS3 release dates for Dragonborn, Hearthfire and Dawnguard - Euro Update/@comment-166.137.156.18-20130207190351/@comment-4057437-20130209043447

Dawnguard has two quest lines, adds two new skill trees and a new form of vampire as well as Fort Dawnguard, the Hidden Vale and the Soul Cairn.

Dragonborn has only one quest line and Solstheim plus all the parts of Apocrypha visited aren't all that large. Adding sea travel and riding dragons probably made the file size a lot larger than it otherwise would have been.