What you are describing does not account for the additional thickness necessary in the larger bottles in order to hold the greater mass of product without the bottle leaking/cracking.

Weigh the empty bottles. Weight equals total amount of plastic and includes the thickness differences plus the spout/dispenser which is what cause a lot of the extra weight. Waste is measured in weight. These larger bottles that are being replaced and thrown away when empty cause more plastic waste than the little bottles did, if you are measuring the plastic waste in terms of weight.

Oh and the next argument is, well, they refill the large bottles. Actually not true in many cases the large bottles are disposed when empty "for sanitary reasons." No, that is for employee convenience reasons. But if they do refill them what does the product that they refill the large bottles come in? Another plastic bottle... an even bigger thicker one (really, really thick- go to a hotel or restaurant supply house and see)... same story...