A tall latte and a venti latte differ by about 130 calories when both are made with 2% milk — roughly 150 versus 280. Most of that gap is simply more milk. Where it gets interesting is sweetened drinks: syrup pumps usually scale with size too, so a venti can carry double the sugar of a tall, not just a third more.
Iced drinks hide the effect better because ice displaces liquid, but the syrup still scales. A grande iced coffee with milk and classic syrup sits near 90 calories; the venti version lands around 120–140 depending on how generous the pour is.
The practical takeaway is not 'always order the smallest'. It is that size is the single biggest lever you control at the counter — bigger than swapping milk types, bigger than skipping whipped cream. If a drink is a daily habit, one size down is the quietest calorie cut there is.