And therein lies the issue, how clear is clear?

For example, if someone managed to get hold of bottles with slightly thicker glass, you could sell a bottle of wine with slightly less wine in than is obvious from the outside, increasing the price per mililitre by a few percent. Not much individually, but it all adds up over the year.

You put the volume on the label, like you already required to do along side the ABV and other markings. That tells you how much liquid is inside - not trying to judge the size of a bottle by how thick the walls are.

Standardisation simplifies manufacturing (of bottles) as well as purchasing of the end product by consumers. There is no benefit to an overly wide selection of sizes.

This makes no difference to manufacturing really. If it did then all bottles would be the same shape. We can have different shaped bottles for everything already so varying the size makes no practice difference here.

These arguments for standard volumes of bottles are very weak. There might not be any big benefit to different sizes, but there is also not a huge disadvantage either. At best it is mildly simpler to compare things of the same size rather than just at a price per 100ml (regardless of the actual volume). Though you should still have a price per 100ml so you can compare the cost of things at different sizes groups (even for the same product).

A far better argument against this is that it is a pointless stupid waste of time that no one asked for and no one under the age of 50 was even alive to remember wine being sold by the pint. There are far more important things the government can be spending their tax payers money on fighting for.