Textures aren't that big compared to all new meshes, animations, creatures, huge new areas, etc. (which Dawnguard had a
lot of)
If Dawnguard is around 500, then Hearthfire will likely be around 100. Since it doesn't seem to have any new creatures, spells, etc.
Just new food/construction meshes, cooking animations... smaller things like that, compared to vampire lords and Soul Cairns.