In the MIDI Track View, you have to put the mouse over the Piano Roll (in the Track View, on the left side of the content area) and move the mouse whilst you press the Left button to Zoom and press the Right button to Scroll.
 
In the PRV it's Mousewheel, Shift+Mousewheel, Ctrl+Mousewheel and Alt+Mousewheel, and combinations of these.
Which doesn't always work very well, sometimes I have everything squashed up in the left and it won't expand horizontally anymore.
 
But the point is, there are two totally different ways to do the same thing.
Could these be unified in a future version?