This roaster has realy got you confused.
There is no real relationship between batch size and roast time. Roast times will vary due to roaster capabilities, results in the cup and the bean to be roasted.
Coretto at home, 300g, around 16min
5kg at work, around 15:30 min
15kg at work, around 17min
Sample roaster, around 8min - but these roasts are for a general "tasting" profile and to check for defaults before purhasing any coffee.
Another local roaster, 20kg, around 14 min.