Lens focus breathing. Ever heard about? According to Wikipedia:
[Lens] Breathing refers to the shifting of angle of view of a lens when changing the focus. Some (often higher quality) lenses are designed to lessen the degree of this effect. Lens breathing does not prevent one from racking focus or following focus with this lens, but it lessens the desirability of any type of focus adjustment, since it noticeably changes the composition of the shot. This is not to be confused with the suction and expulsion of air from within the lens as its internal volume changes.
Want it easier? DPReview TV to the rescue: