I'll risk pooing on internet knowledge
The chamber is open at low RPM as that is where the pipe is out of phase with the engine speed. On a conventional engine the returning waves are disruptive off freq as they are returning too early on a wide open port. The chamber open disrupts these returning waves. Typically a chamber size somewhere near the cylinder volume will work well enough but isn't critical.
Of course it would cost GG $0.2c to make bigger covers & I doubt they would make the same mistake year after year.
Most reports are anecdotal & involve other changes.
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