Because it's bigger?
We played 2 seasons at Wembley, which is almost exactly the same size, and we still played well.
When we were good, we were winning away at pitches more or less the same size. We were doing that at the start of this season before the wheels fell off.
Its not the pitch size. For me, our home woes are because the players started fearing playing there or, at the very least, never looked forward to it.
Injuries have obviously exacerbated the situation, but it's a mentality thing IMO.