More layers of line, larger diameter.I believe the load capacity reduces with additional wraps not because the winch has to pull the line in faster but because the diameter is larger and therefore the ratio between the input shaft rotation to winch cable rotation changes (like gear ratio).
Larger diameter, larger circumference.
Larger circumference, more line that has to move per revolution of the winch.
More line that has to move per revolution of the winch, the faster the line has to travel for a given PTO speed...
They are all mathematically related (circumference = 3.14 x diameter), so any of the above is as good an explanation as the other.
-Chuck