Thank you for the reply. I assumed it might have had something to do with dpi. I did right-click the images to look at the dpi values, but I don’t know what dpi really means or how it’s used. I just started using Sumatra to try to read manga. The first one I read scaled as expected, but this one didn’t.
So after thinking about it, it appears setting CustomScreenDPI is exactly what I needed. The dpi for all images in a manga volume should be the same, so it only needs to be set at most once before reading each volume. Setting CustomScreenDPI to the same as the jpeg’s dpi gives the “actual size” that I would expect, but I can just pick an arbitrary value to get just the right amount of zoom. (I don’t actually need to view the images in “actual size” because that’s just an arbitrary size.) So with the press of a single button (using AutoHotKey to remap the shortcut) I can zoom in by a specific, desired amount. And another button can be used to zoom back out (fit to page). I just have to set CustomScreenDPI beforehand. I used to use IrfanView to read manga, and this is something that didn’t appear to be possible. (In IrfanView, there’s a “display multiplier %” for fullscreen mode, but changing it causes “fit to page” to no longer fit to page.)
So as far as I can tell, SumatraPDF is possibly the ideal program for reading manga. I’m switching from IrfanView after getting a larger monitor because IrfanView can’t display 2 pages at a time. With AutoHotKey, all the necessary shortcuts can be remapped to single keys on the numpad. (I have + for fullscreen, - for bookmarks, end for “actual size”, ins for “fit to page”.) I also have the left and right arrows on the numpad swapped, so I can advance to the next page using left instead of right, which feels much more natural when reading right to left. Sumatra doesn’t hide the mouse cursor in fullscreen like IrfanView, but for that I’m using a separate mouse hiding program, which can be started and closed thru AutoHotKey with a single button. (edit: This last one doesn’t work quite right at the moment because of windows antimalware, but anyway… I’ll have to do some searching to see if there’s a better alternative to Sumatra or not. The lack of mouse hiding seems to be the only thing missing that’s needed.)