So...
In raw sizes, the HD atlas sprites should be double the size of the SD atlas sprites. A sprite that's 50x50px in the SD atlas, is 100x100 is the HD.
If we keep a fixed size of 500, and position the sprite at 0,0 then that sprite from the SD atlas should take up 1/10th of the screen height.
Changing to HD atlas, with no further modifications, makes the new sprite take up 1/5th (double the size, since it's 100px high).
So to offset that, we change the atlas to have a pixel size of 0.5f, such that it will still take up the same space on the screen 1/10th even though it has 4 times the information in it.
You could do it the other way around and design with your HD atlas in mind, keep that at pixel size 1, but then SD atlas needs to have pixel size of 2.
Like ArenMook said, you're doing the opposite of what you're supposed to be doing. The higher definition atlas, needs a lower pixel size, since you're cramming more pixels into the same virtual space.