<Given the same atmospheric conditions, equipment, and process, you will boil off the same volume of liquid whether you are boiling 2 gallons or 10. Variations in boil-off volumes are determined by the aforementioned factors of atmospheric conditions (ambient temp, barometric pressure, wind, etc.) , equipment (wort surface area in your boil kettle, lid or not, etc.), and process (duration to get wort to boil, duration of boil, boil vigor, etc.)>
I think you hit the nail on the head with this statement.
The biggest factor in liquid loss in this case is probably the equipment "(wort surface area in your boil kettle, lid or not, etc.)". The amount of surface area will affect the amount of evaporative loss and drift to a large degree, that and the lid will make a big difference.