It is easy to see the larger instrument will see through a greater variety of air temperatures and will encounter more moving, mixing air. This mixing air will retard and advance the wave front and render the image quality very poor. Recall how Jupiter or Mars behaves in a telescope of this aperture. As single cells of mixing air move over the aperture the entire image will shift about with details intact. The image in the larger telescope will jiggle about and show less surface detail.
Atmospheric turbulence is the great equalizer.
For years I have used three or more telescopes of different aperture on any given night. For a really excellent review of tube currents, something particular to open tubes, see Understanding Thermal Behavior in Newtonian Reflectors, Bryan Greer, Sky and Telescope September 2000. In the next Constellation I will give directions on building and using a knife edge eyepiece on the stars to further explore tube and atmospheric currents. Having just finished a series on eyepieces, and being at a loss for words, I'll talk on a topic mentioned by Brad Miller. He recommended a series of formulas for various telescope applications, so here goes.