Lightroom CC 2015.8 and Up
Adobe finally fixed this in Lightroom CC 2015.8. The Fit mode in the Zoom section of the Toolbar will now stretch low-resolution images to fill the view area without clipping:
If you can't see this, just press Z once or twice to toggle between your current zoom level and Fit mode.
Also of note is that the so-called 1:1 zoom level mimics a non-HiDPI screen, in that the image size will be the same on-screen as for a half-resolution screen of the same size, at least here on my 5K iMac. I have to select the 1:2 mode to get a pixel-for-pixel display.
I infer from the release notes that this feature was also added to Lightroom 6.8, but I do not have a copy here to test.
Prior to Lightroom CC 2015.8
Prior to this release, the justification for Lightroom refusing to scale up low-resolution images to fill the view area is that photos should never be enlarged past 1:1, since this reduces the quality of the photo.
I disagree with this, since it makes the photo look no worse than it inherently is, being low-resolution.
Fortunately, there is a tolerable workaround: Use Survey view (N) instead of Loupe view (E).
This mode was made for working with multiple images; it behaves a lot like the Compare view mode (C). One big difference is that when there is only one photo selected in the Grid view or Filmstrip, going into Survey mode doesn't show the next photo alongside the one you had directly selected. In that respect, it behaves more like Loupe view, except that it will use all the screen real estate, even if that means stretching the photo to fit.
This trick has a few problems:
It doesn't work in full-screen view (F).
It uses a fade-thru-background effect when moving between images, which slows you down when scrolling through multiple images. The regular Loupe view switches images instantly.