Unit cost of the 8oz bottle=8/16=$0.50 per ounce.
Unit cost of the 128oz bottle=53/128=$0.41 per ounce (approx).
So the larger bottle (128oz) has lower unit price.
Another way of looking at it is to note that 128=8×16 so if the unit prices were the same the larger bottle should be 8 times the cost of the smaller bottle: 8×$8=$64, but it's only $53, is it's cheaper according to the unit cost. The arithmetic here is easier than calculating the individual unit costs.