Question

All Starbucks cups hold the same amount of liquid

Answer 1

The claim that all Starbucks cups hold the same amount of liquid is an incorrect statement. People on apps such as TikTok use a tactic called "Click Bait" to gain more views. These videos bring them revenue, so they must use something shocking. Each size has an allotted volume: Tall 12 oz, Grande 16 oz, Venti 20 oz hot and 24 oz cold, and Trenta which is 30 oz. 

Source: Fact or Fiction: All Starbucks cups hold the same amount of liquid? (10news.com) and personal experience 

Answer 2

From the video, we cannot determine if there is anything occupying volume in the other cups. It is clear that this video was deliberately made to mislead the viewers.

At Starbucks, the drink sizes are generally divided into four categories: Short (8 fl. oz./236 ml), Tall (12 fl. oz./355 ml), Grande (16 fl. oz./473 ml), and Venti (20 fl. oz./591 ml), corresponding to the Italian words "short," "tall," "grande," and "venti." It is worth noting that the Short size is typically available for hot drinks, while cold drinks (including Frappuccinos) do not have a Short size option.

For cold drinks, such as Frappuccinos, the Tall size is 12 ounces, the Grande size is 16 ounces, and the Venti size is 20 ounces. For hot drinks, regardless of the size — Short, Tall, or Grande — the volumes are 8, 12, or 16 ounces, respectively, with the Venti size also being 20 ounces.

Starbucks Cup Sizes (Explained): Grande, Venti, etc. | Trusted Since 1922 (rd.com)

Confused By Starbucks Drink Sizes (And Cup Sizes)? We Explain | Drink Scouts