Valo theme on iPhone

The Valo theme demo works fine on my iPhone. I created a new app from this demo. I puted a login screen into it. When I resize the browser window on desktop, the responsive layout works fine, switch to tablet size and phone size. But when I test it on my iPad and on my iPhone, it switch to the table size only, and not to the phone size. 

Can I get any idea what may is the problem? Why works the tablet size and not the phone size? I’m pretty clueless :(

Maybe you have also issues related to mobile browser viewport size as discussed in this thread https://vaadin.com/forum#!/thread/3993515