Zygomatic arch - Wikipedia, the free encyclopedia FYIOK, I will ignore the last post, and assume that your mouthpiece is attached to a horn at the time of buzzing. When buzzing the orbicularus muscles are being pulled by the larger straps of lateral dilating muscles in the faced (kind of like a forced smile feel). This tends to narrow or close off the aperture as that smile is tensed. That is likely what you are describing. So one piece of advise is not to put so much tension on those muscles when buzzing.
I actually don't like to buzz and rather use a pwhooo (soft vibratory) force on the mouthpiece rim. I get that by using the more diagonal dilating muscles attaching to the orbicularus, that anchors above onto the zygomatic arch (the bone just below the orbit of the eye). That takes the face into a curved coroners up smile. It not only opens the aperture more for air outflow, but is also creates the more socially acceptable smile that is loved by the ladies.