(The following concerns extended objects only, not point sources.)
This myth of contrast increasing for extended objects with increased magnification due to the darkened sky is still making the rounds. It's a particularly pernicious, antibiotic-resistent bacterium of an anti-fact.
Boosting magnification shrinks the exit pupil, which in turn dims both sky and object equally. Optically, contrast does not alter in the slightest. The increased image scale affords improved detection of detail, which very powerfully gives the *illusion* of improved contrast.
In fact, the scene dimming via exit pupil shrinkage actually *reduces* contrast because visual system noise becomes relatively larger. But in spite of this dimming and worse visual noise, the image scale increase involves more retinal cells which can extract more information...
...until a point is reached where a reversal, or turnover occurs, and visual system noise's contribution exceeds the gains afforded by increased image scale. Depending on sky and object surface brightness/contrast, this turnover occurs over a good range in exit pupil diameter. The brightest planetary nebulae can withstand surprisingly small exit pupildims, whereas the very low surface brightness nebulae in a dark sky demand at least moderately large exit pupils.
The persistence of the myth of boosted contrast for nebulae and galaxies via increased magnification must derive in part because of the *power* of the illusion imparted by the gains afforded by the increase in image scale. Even if an observer has heard or read of the true workings, the impression gained by his own eyes would seem to deny this, and instead support the contrast increase misperception.
Some might say, "If the net effect is like a boost in contrast, why get wrapped up in the details of why?" Well, this goes to understanding and differentiating between the external optics and the human visual system. I think it's important to know which does what, for it then allows us to more effectively use our equipment.
I've been battling this myth for decades now, and wonder if I'll live to see its expiration/eradication. I'd rather go out feeling like a St. George that a Don Quixote.