there are a number of issues with the QT version we use in the player. The version is quite old and the reason we haven’t upgraded is that this version is well tested and carefully trimmed down to reduce the size, since we only use it for the GUI. Unfortunately this version has incomplete support for the retina displays, besides showing a number of problems when trying to resize the GUI on the fly (this explains why, differently from the web-player in the Portal server, the desktop player doesn’t adjust to fit the window size). The Android and iOS players use the latest QT and, given the code base is the same, we expect to complete the transition as soon as these players are released. This should help to solve all these problems.
About the resolution of the NX session itself, you must consider that while are the lower layers to handle encoding/decoding, the final stage of showing the pixels on screen (on Mac) is still handled by the QT library, so even if the remote screen has an higher resolution, the window on screen will be displayed at 1440×900.