I'm in the UK and the only network offering unlimited data is so slow you'll struggle to use more than 3GB in a month.
I've since given in and just paying through the nose for 16GB/month, so that I can not have to watch my data usage. (Un?)fortunately, the network is so fast I've found my data usage is already around 9-10GB/month, so I'm going to have to start watching that again soon.
Apart from laziness there is no reason for those websites to be so large.
Sure there's always room for an amount of optimization and good practice but it's insane to rely on that instead of fixing the problem. The growth in page size has been fairly linear and predictable.
The real headline here should be "German and US carriers cannot keep up with natural growth in technology"
You make it sound as if the natural growth in technology is what makes these websites huge. I run a web app, which is uptodate technology-wise. It has a responsive design with @2x images and @3x. It uses JS in a sensible manner. It's fast and easy to use. Top notch technology. Yet, the average request sums up to 200KB. And 60KB of that is a custom font.
This is not about the natural growth in technology, it is about 50-70% of the 2MB are entirely worthless to the user, because they are usually Ads, trackers, wrongly compressed images (ImageOptim does wonders here) and using 10 JS frameworks in parallel.
It's almost as if you're saying: See, img tags now support the width attribute, so let's upload a 10MB photo and just tell the browser to resize it to 100px, because technology.
So I go with a "those in glass houses" approach:
The only reason I can't see the terrible quality of your website's photos on my phone is that all the content is so damn small and zoomed out I can barely make out the text never-mind the picture pixels. I don't really want to zoom in because the shade of lime green you've chosen is so jarring it'll likely give me a migraine if it fills the screen.
Users, regardless of device used, are primarily on wifi and that is what is designed for. In modern countries most users don't really have data restrictions low enough to come into effect through websites so going into the future this will not be a problem.