But the origin is size inflation. The original Starbucks sizes were short and tall. Shorts are still available, in at least a few outlets, despite having not been on the menu for at least 20 years. (For a while a few years ago here in the UK, I even saw short as a column on the menu at one, but that's very rare.) Then they added grande, literally meaning large but maybe meaning more like "extra large". Then when they added an even bigger size, since large was already taken, they just stated the actual size "venti" = 20 (fluid ounces).
(The first time I ever went to a Starbucks had the same confusion as you. I didn't know what an espresso was but at least its sizes of "single" and "double" were easier to understand, so I got a double and was a bit surprised at how small my drink was!)