Forgive me, I was in error regarding the can size. I assumed the tiny cans were 100ml, but they’re actually 8.4oz , which is around 250ml.

The reason red bull uses the 100ml number is because it’s a rate that can be applied to the different sized cans.

Also, the reason you want to use serving size instead of equal volume is because you consume different things at different volumes. Consider alcoholic drinks. A pint of beer, a glass of wine, and a shot of liquor are all significantly different volumes. But the serving sizes all have similar amounts of total alcohol. No one should be drinking a pint of straight vodka.