Coffein simply blocks the receptors in your body that send the signals to your brain that you are tired, while sugar is essentially pure energy. I'm no expert on this subject, but afaik both coffein and sugar increases blood pressure, which is probably not an ideal thing.

I dont think Starbucks is considered a cofee in europe. More like an american desert cofee honestly. And how large even is that? Half a liter? Typical cofee in europe is at max half of that.

There are huge variations between chains in a single country, let alone between countries. And 'large' in the US likely means something very different to what 'large' means in different parts of Europe.

But your point stands up, in general. Starbucks is noticeably low in caffeine (in the UK) and 90mg would be low in a medium (chain) coffee here:

A medium cappuccino at Costa Coffee contains a “massive” 325mg of caffeine, almost five times the strength of the Starbucks version with a modest 66mg.

By contrast, Greggs and Pret A Manger also use significantly less caffeine in a cappuccino of the same size, at 197mg and 180mg respectively. Caffè Nero had the second lowest levels of caffeine after Starbucks in this drink specifically, containing between 110mg and 115mg.

The study also found that one single espresso from Pret A Manger contains 180mg of caffeine, or six times as much as its Starbucks counterpart. Pret’s filter coffee also contained the highest caffeine levels at 271mg, compared to 225mg at Greggs and 102mg at Starbucks.