Mirrorless Flange Distance

[Guest]

I keep hearing that mirrorless camera lenses will beat SLR lenses because of a smaller flange distance. I don't understand why having the lens closer to the sensor will improve image quality. I can understand why the image may be brighter for a given f-stop, but am still not understanding why there would be an optical advantage. If anything, not allowing light distance to spread would mean needing to design larger lenses that still cover the sensor at a diminished flange distance. Am I wrong?