Large expansion or Small expansion

[keglo]

Can someone tell me the difference between a Large expansion and a regular expansion? I'm talking about the hero pack expansions. For example Raknar's Vision: Heroes of Lindesfarme is considered a large expansion, while Heroes of Nastralund is considered a small expansion. As far as I can tell they both have the same number of heroes (5). What makes the first one large? The actual size of the figures? Or does it come with terrain? I don't get it.