As helpful and essential as the mind is to drawing, sometimes it gets in the way. One way in which this happens is the overawareness at some subconscious level of the edges and constraints of the page while drawing. Quite uncontrollably the mind tries to compress the drawing so that it fits into the space available regardless of what this does to proper proportions. Though simply letting the figure flow off the page and cropping whatever doesn't fit is the proper approach, unless you force yourself to do this its not something that happens naturally. Supposedly this habit is something that is learned as you get older, as when young children draw they have little regard for the boundaries of the page. Or at least so it said in some book that I read at some point and have mostly forgotten.