One of the other reasons might be to keep the TWY clean of debris.
While A380 might not receive any FOD during taxi it still can blow quite a lot of stuff onto TWY with 1 & 4 (even if the edges are clean, you never know) if it is not wide enough, in that case I believe TWY check will have to be performed by responsible personnel after A380 taxi, therefore probably delaying other a/c.