Starbucks use Arabica, typically Italian or French espresso based drinks ha e been made with robusta or an Arabica robusta blend.
Starbucks coffee is not higher in caffeine compared to other cafés, it can actually be lower. The only sig ificant difference is portion sizes, like you mentioned.
Here are some people writing about their experience of drinking 100% Robusta: https://old.reddit.com/r/Coffee/comments/6u2ef4/100_robusta_...
Starbucks coffee doesn't really have a higher ratio of caffeine, just more volume than the official measurements.
That being said, I don't know anybody that drink 8oz cups of coffee.
https://www.triplebarcoffee.com/blog/how-many-ounces-are-in-...
A Starbucks dark roast (short size) has 130mg at 8oz - that is 16.25mg/oz.
That means the espresso has 4.6x the caffeine per volume. I’d say those people you’re tired of have something worth listening to.
https://globalassets.starbucks.com/assets/94fbcc2ab1e2435985...
Starbucks drinks typically have 2 shots (such as a grande mocha) which gonna be around the same amount of caffeine as a cup of coffee
I doubt Starbucks is deviously out to over-caffeinate its customers (although if they were, they've got my vote!) but rather the difference is attributable to variations in their brewing process.
There's a limit to the amount of caffeine that can possibly come out of a bean, and so I would say that unless there's evidence they're spiking the brew, that Starbucks coffee is roughly the definition of an "average cup of coffee"
A Venti is 590mL and 415mg of caffeine, 700mg/L of coffee. Based on the chart in [1] that would be roughly speaking the midpoint of roast kind at the 150 minute mark making cold brew, 10g/100mL of coffee beans (Figure 1). Or approximately a dark roast, coarse grind hot brew (Figure 3).
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5740146/