The footage so far has all painted a genuine open world, not a corridor fest ala Crysis 2, if that's what you mean.. Redfall's world is
27k square meters, i-e around 17 square miles. For reference, Skyrim is 15 square miles, and despite Ubisoft boasting a 1 : 1 recreation of DC,
the playable areas of The Division 2 are 11.4 square km (around 5 square mile if google is to be believed).
It's a pretty big game world and un-tethered 4 player co-op would naturally incur some cost. Cost enough that they haven't been able to get it to 60 at launch but will likely make necessary CPU culling related cuts to get it there via updates.
And once again, Back 4 Blood is not even a viable comparison. It just feels like you're purposefully making that comparison because of some perceived similarity in the games art styles.