The authorities are saying they don't want to see any trash at all, regardless of volume. Imagine 100 sheets of paper vs 100 AA batteries. The batteries have much more volume, but the sheets of paper cover a much larger area so there's much more visible trash.

Not much difference between a 12" and a 18" lag bolt for the purpose of "how much trash is visible and impacts terrain".

Surface feels a bit fairer in that sense. Or at least, easier to measure.