The design canvas in design mode doesn't give you an exact representation of the design, in app previewing gives you the same results as browser so this is all right.
While you build, design mode may be slightly off. Preview mode gives you a closer rep.
As for the max width, try a new project to see if you get same results it may be a bug with 2.4.1, or a conflicting class in your project.
BTW welcome and thanks for your support! You may spot a few RW users here in the forum