Quoting the developer: "...the game will run in 100% size (with option to scale it up or down) so the backgrounds will be perfectly sharp. 1250x790 is usable resolution for screens 1280x1024 or 1280x800 or bigger and you can still play it full screen or in window without scrolling. also we don't mind small black frame around the screen (in full screen mode)."
When i scaling manually with system Screen Zoom (ctrl+mousewheelup/down) game looks OK.
Looks like you found a really useful fix that other MAC users may use one day. Thanks!