On the one hand it shows that the items are not included as part of the game's main installation and simply unlocked when you purchase them. There's no clear reason why these items take up so much space.
You'll also need 1.5GB of free hard drive space on Xbox One or 2.1GB on PS4. It costs £3.99 or is covered by Syndicate's season pass. Then there's the Steampunk Pack, which includes Jacob's steampunk costume plus a few accessories (a belt for him, a cape for Evie, plus a new knuckle duster, gun and gauntlet). It weighs in at 820MB on Xbox One and 1GB on PlayStation 4.
Ubisoft has released Evie's steampunk outfit for free. You can now dress Assassin's Creed Syndicate's leading duo in a set of steampunk clothes - but you'll also need up to 3.1GB of hard drive space for the fresh wardrobe items.