Corsa silencer question
 

I recently bought a boat that has these silencers installed on a Mercury 525 efi. I removed them to check them out and inspect for corrosion. These look extremely restrictive to me. There's only a little over an eighth inch of space around the last plate that's welded in. When I look inside from the front end I get even more confused as to how these flow enough exhaust alone exhaust with water mixed in. Can someone tell me what I'm missing? I see them all over so I'm sure they work but it doesn't make sense to me.