.Big Files are files that edit the look(Textures) of objects and creatures in the world of fable.
How to Make:
After adding a texture into Fable Explorer right Click on "Textures.big"
Select "Save changes as Big"
Save it as What you want
How to add .BIG Files to Fable:
Right click on "Textures.BIG"
Select "Merge .BIG"
Select the file you want to add
Click "Save mods and run Fable"
This should help for modders and just average gamers.