A cocktail recipe travels badly between countries, and the reason is the measure rather than the ingredients. Bar measures were standardised nationally for legal reasons, and the numbers do not line up.
Legal serving sizes came first
Many jurisdictions regulate the size of a spirit measure sold in a bar, because duty and consumer protection both depend on it.
Those regulations were set separately in each country, at values that reflected local custom rather than any shared standard.
The equipment followed the law, so jiggers and optics in different countries pour genuinely different amounts.
Recipes written in parts survive translation
A recipe expressed as two parts spirit to one part each of citrus and syrup can be made anywhere with any measure.
The same recipe written in fixed volumes becomes a specific drink at a specific size, which changes when the measure changes.
This is why historic bar manuals and modern professional recipes both tend toward parts, while consumer recipes use volumes.
Glass size sets the upper limit
A recipe has to finish inside the glass it is served in, including the water picked up from ice.
Scaling a recipe upward by using a larger measure can overflow a standard coupe, so the glassware has to scale with it.
Glass sizes have grown over recent decades, which is part of why modern versions of old drinks are larger than the originals were.
Kitchen spoons are not bar spoons
A bar spoon is a stirring tool with a bowl at one end, and its capacity is smaller than a kitchen teaspoon and varies between designs.
Recipes calling for a bar spoon of a strong ingredient are giving a rough dash rather than a precise volume.
Where precision matters, professional recipes give millilitres for those ingredients instead, which is a reliable sign the quantity is doing real work.
Dashes and drops have no fixed size
A dash of bitters depends on the bottle, the dasher insert and how hard it is shaken, and can vary by a factor of two between bottles.
Bars that need consistency count drops from a dropper or measure bitters in millilitres for batched preparations.
For a single drink the imprecision is usually tolerable, which is why the convention has survived long after every other measurement in the bar was standardised.