Not quite, Windows uses hinting information from the font, OSX doesn't. Many freely available fonts contain terrible or no hinting, and they will look bad at small sizes on Windows.
IE9 promises to not do this anymore, and on high-density displays (the better smartphone ones) it is simply not needed.
So the problem is two-fold: a very distinctive look of Microsoft fonts, and a sub-optimal fallback for non-aggressively hinted fonts in Windows.
However, if one uses font-sizes that one can actually read (for instance, bigger then 12pt, thank you all very much) the pixel-per-character count gets big enough that the results start looking better -- especially as the user does not unconsciously move his nose to meet the display in person.