Breakpoints pixel width

Hi, seems like I'm missing something in my understanding of how the breakpoint actually functions. I've got a project with a mobile breakpoint of 414. This is the version that is displayed on my iphone 6 in portrait mode. However the actual pixel width of my iphone is supposedly 750px so I should be seeing the tablet version. Thankfully this is not the case, should I rather be looking at the uikit size that is mentioned and is this a common feature on mobile? 

I'm also wondering if it would be a good idea to increase the mobile breakpoint to e.g 600 px to provide a larger canvas that would provide better resolution if the user wants to zoom in?

Thanks Jorgen