Thanks for the answer, Mikero. That all makes sense.
When I said that "it looks as if the operating system were trying to simulate a high resolution on a low resolution display", I was actually referring to the situation in which the option "Use full resolution for Retina display" is disabled. The high resolutions are available inside Ubuntu, but it seems that the virtual machine is limited to a lower resolution. The operating system emulates the high resolution in terms of size, but the images are blurry. I don't think this problem is relevant, though. In this situation, I don't think the high resolutions should be available at all.
When the option "Use full resolution for Retina display" is enabled, things behave the way you described. The virtual machine has full access to the high-density display and the image becomes sharp and tiny. Indeed, Linux is not fully prepared to work with a high-density display. I've done a lot of research and found that Gnome Shell works a little better than Unity (specially if you tweak the font-scaling factor and use large icons). However, this setup is not good enough to be used on a daily basis. Maybe the next versions of Ubuntu will have better support for high-density displays.