Straightforward enough, I think. As soon as you go fullscreen or resize the window, the image resizes to fit the width.
Thank you for reporting this bug in KDE software. As it has been a while since this issue was reported, can we please ask you to see if you can reproduce the issue with a recent software version? If you can reproduce the issue, please change the status to "REPORTED" when replying. Thank you!
I no longer use Peruse, but as far as I can tell, the version I just installed (1.80.0) no longer supports fit-to-width at all, so this bug is no longer relevant. For clarity, fit-to-height is when the top and bottom of the pages are glued to the top and bottom of the window, and fit-to-width is when the left and right sides of the pages are glued to the left and right side of the window. With fit-to-width, you'd usually need to scroll up and down to see the full page.
Dear Bug Submitter, This bug has been in NEEDSINFO status with no change for at least 15 days. Please provide the requested information as soon as possible and set the bug status as REPORTED. Due to regular bug tracker maintenance, if the bug is still in NEEDSINFO status with no change in 30 days the bug will be closed as RESOLVED > WORKSFORME due to lack of needed information. For more information about our bug triaging procedures please read the wiki located here: https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging If you have already provided the requested information, please mark the bug as REPORTED so that the KDE team knows that the bug is ready to be confirmed. Thank you for helping us make KDE software even better for everyone!