When i have a normal short, say 1-page PDF it views perfectly in DT3:
I select it and it shows automatically as Zoomed to Width (because I have set the PDF Display setting to “Single Page Continuous”).
But then I select a loooong 260-pages PDF – and it shows rather small as if I had selected "“Single Page”. Even hitting manually Ctrl-Cmd-W does not change this.
Not sure, but that happens when the PDF has a bigger page (landscaped, for example) and it adjusts to the biggest page. At least this was the reason when it happened to me.