When you set the resolution in main menu options and then either load a game or start a new game it reduces the resolution each time.
Do you mean you are setting a higher resolution than the default setting, and then the resolution is resetting to default every time you load or start a new game. Or is the resolution getting smaller and smaller and smaller every time?
I will assume it's the former, in which case this is a known issue that Tynan has said will be fixed in the next update.
https://ludeon.com/forums/index.php?topic=28308.msg286514#msg286514
If it is the latter though, then please can you post log file as an attachment.
This happens to me as well. I set the resolution to 3840x2160 in the main menu with the UI scale set to 1.5x. When I load any save game, or begin any new game, the resolution will change to 2560x1440. Similarly, if i begin with 2560x1440 resolution, it will reduce to 1360x768. Changing back doesn't seem to be a problem once in game, just a minor inconvenience. No matter the resolution I choose it will always be a bit lower once I begin a game. Not sure if the UI scale setting makes a difference or not but I felt it would be helpful to include either way. OS: Windows 10 V.1607. Game Version: 0.16.1393 rev533. GPU: NVidia Geforce GTX 1080.
Already fixed for next build.