It's resampling to the size of your output from the size of the LCD.
For example, you can run at 16000x12000 if your vid card allows it, but it's all being shrunk down (by the card or by the monitor hardware) to fit the physical pixels on the LCD itself.
FYI: Text in the buffer doesn't use the hi-res pack. The hi-res pack is textures. The text uses a type of a font.
You can change the size of your text in the settings. The radio bar is "fixed" in size, if I recall. So if you up the resolution, then up the text size so it looks the same, the higher resolution means your radio box is smaller (same pixels, more density of pixels, means smaller object)