The 4:3 7.85" Mini is 4.7" wide (portrait orientation).
The 16:10 7" Nexus 7 and Kindle Fire HD are 3.7" wide.
The web page is scaled to fit horizontally with no text reflowing or other formatting changes, so it's like pure zooming.
3.7/4.7 = .79, which is the scaling factor between Mini and 7" tablet in this scenario.
To keep it simple, assume a character is 1" wide on the Mini. The Mini obviously has 163 pixels to render this character. That same character will be only .79" wide on the Nexus, which thus has .79*216 or 170 pixels to render it. So the Nexus can't render the glyph with much more fidelity than the Mini in this scenario, which applies to any document that is scaled to fit with no reflowing of text, such that the only difference is the apparent magnification.
In general, instead of having 216/163 or 1.33x the pixel density of the Mini, the 7" tablet really has only 170/163 or 1.04x the pixel density in this scenario. Doing the same thing for landscape mode, the Nexus improves to 204 PPI while the Mini stays 163 PPI. However, looking at it from a different angle, rotating to landscape does perform an implicit zoom, the text will be 1.33x bigger when the Mini is rotated, and readability benefits because the text will be bigger and rendered with more pixels, so it's not like the Mini doesn't improve when you rotate it to landscape.