It is generally thought that Starbucks named their sizes differently to set themselves apart. Much to the chagrin of local coffee shops, customers often request a Grande or Venti when ordering. These are not generally accepted cup sizes at other coffee shops!
Most of the names Starbucks uses are Italian. This plays on the idea that visiting a Starbucks is like a trip to Europe or at least a quick virtual European experience.
Knowing how much coffee you’re getting in each size can help you figure out how much caffeine is in your Starbucks drinks. In the case of sweetened drinks and those containing milk and chocolate, size is a big factor in how many calories and how much fat you’re drinking. Calories are also listed on the Starbucks menu for most drinks, so you can get a good idea of what you’re drinking before you accidentally down a 500-calorie Frappuccino.