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Speaking specifically about imaging setups, several scope manufacturers have increased secondary sizes to provide "full frame" camera sensors with larger unvignetted fields. In the past APS-C sensor sizes were the norm, and secondaries were designed with that in mind. A good example is what Skywatcher did to the venerable MN190 Mak-Newt. Their version of this very good scope had a 52mm secondary, which was perfectly adequate in terms of APS-C sensor coverage. The secondary on a Mak-Newt cannot be moved (they are mounted directly on the rear of the corrector, and offset is fixed by design), and focal length cannot be adjusted. With the advent of "full frame" cameras vignetting became an issue. Skywatcher solution was to increase the size of the secondary to 63mm, to provide a sensor field adjustment with less Risk of vignetting. So all new MN190s now come with 63mm secondaries. Of course this impacts central obstruction, and does degrade the scope slightly in terms of visual use. However, it was obvious that Skywatcher hoped to sell more of these scopes, to full frame camera owners, and avoid questions about vignetting. As a former owner of a small secondary MN190, I regret selling mine, as they are now collector's items, and imho out perform the larger secondary versions.
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