Prior to calibre 3.0 calibre displayed in more tight, compact look. I had 16 icons with text on main bar, now only 14 of them fit. Also book details pane now displays larger white spaces between lines and much less text is being displayed without scrolling. The amount of books displayed was reduced too, but I managed to adjust it by setting spacing to low minus values.
I read here that turning off that new high resolution option ("Adjust for high resolution screens" - is that the one?) should bring calibre to how it looked in 2.+ but that helped nothing - still only 14 icons fitting on screen, also the Book Details section hasn't changed
Due to sight issues I have a setting in Windows to display everything in 125% scale. But I've had that on for years, and previous versions of calibre prior 3.0 weren't affected like that and much more info/text would fit on screen.