SketchUp doesn’t really have a lot of adjustments you can make specifically for line weight settings (not incrementally the way you can in AutoCAD, or the way you might scale the size of a font). To change a line style around you really need to look into changing what Style you’re using within SketchUp.
See… >Windows >Styles, and click on some of the available options. They’re not all visible up front; you’ll want to navigate the pull down menu to access the majority of them.
There are some line weight adjustments, which are available within ‘Layout’. And you can also create your own Style by using a program called ‘Style Builder’. Both ‘Style Builder’, and ‘Layout’ are free companion programs that come bundled into the SketchUp Pro licensing option… and these programs are also something to think about concerning your evaluation of SketchUp. Take a good look at them in addition to the main SketchUp program…
‘Layout’ – specializes in document creation which includes arranging 3d SketchUp models onto a 2D Page format (printed, or digital). Layout isn’t limited to just large project construction documents, it works equally well with single page, or multi-page layouts. and has some nice features which make it easy to create tight continuity, and consistency between multi-page docs.
‘Style Builder’ – as already mentioned allows you to combine your own line work, so that you can create a ‘Style’ for use within SketchUp. Basically you print out a template, and draw in your own line work. Then you scan it back into the program where it in turn converts your line work into a usable SketchUp ‘Style’. (Line weights happen here, by drawing in thicker lines).
Having said all of that. I suspect that the major issue at play here, has to do mostly with your instructors screen capture program, whatever s/he’s using probably doesn’t handle line weights with much finesse, which is to say that it’s very common to see a fine line weight generated from any CAD program get enlarged or scaled to a heaver weight in another program. The same thing can happen in a PDF file depending upon how it’s generated. . . which, in this case, given the line weight settings found within SketchUp’s ‘Layout’ program – you do have some adjustment over this.
Also, screen resolution does affect things, but in addition to seeing line weights change some, you’ll also see the size of menu text, and tool icons get much smaller. Depends upon the settings used on your Macbook Pro. See… >Apple Icon (Top Left) >System Preferences >Displays. . . Resolution – Display (tab):
 ‘Best for Display’ = resolution adjustments happen automatically.
 ‘Scaled = you can adjust slider to a couple of different options.
Hope that’s helpful,