That's because the pixelpixels per inch (PPI) is different between them. The higher the PPI, the higher the quality but at a cost of (relative) smaller screen size. So the nexusNexus 7 has 216 pixelpixels per inch while the galaxy nexusGalaxy Nexus has 316 pixelpixels per inch. The resolution isn't everything, you also have to consider the density of the pixels.
High PPI values have the advantage which is that it's HD but it also has a disadvantage which is a smaller screen size (at a persistent resolution).
You can read more about PPI here.