My phone's calculator app is about 14 MB and the gallery is 10 MB in size. Flutter versions would have been probably a tad smaller than these native Android apps.
Well, just check the file sizes of the apps which you have installed on your phone.
Instead of an actual Crosswalk with trucks of data trying to kill you.
Though I personally have an unmetered 100Mbps connection, my target market(s) for what I develop, are people who are very data sensitive.
A 7MB Hello World file might mean that my app with the world, universe and everything I need, might be bigger. It'd be great if y = m(x) + 7MB with a gentle gradient, but I'm already starting off bad.
I tried creating a hello world with most of the dependencies that I think I would need, but most couldn't load, so I don't know what size a specimen app could be.
Either way, a 15MB download for someone in my radar would mean them not installing the app. One also shouldn't underestimate how many phones in developing countries barely have enough space left. Google Play seems to need minimum 500MB for me to install/update apps on my high-end phone, so it might be dire for lower-end devices.
Also, making UIs with HTML/CSS/JS really isn't that nice. The workflow with Flutter is way more streamlined.
The download can also be an issue, especially when downloading over cell... more so in a rural or other poor signal area.