rdubreuil wrote:
The term generally used to describe this is known as "focus breathing" or "lens breathing". It's a variation in angle of view between the two lenses when zooming/focusing. Higher quality lenses tend to control this effect much better than lower quality lenses, hence why your Canon looks much better than your Tamron. There are many articles on the web about this, just do a general search for focus breathing, I hope this helped you out.
This is the simple and correct answer.