less size means only a couple of pixels are available for font rendering. more the no. of pixels, more the clarity. take the example of an ellipse. the more no. of points u use to draw the ellipse, more accurate the ellipse will be.
about ur anti-aliasing issue - sorry, there's nothing u can do on chrome/opera on windows. ie antialiases fonts. but not chrome/opera. in mac thought fonts get antialiased even at small sizes.
update[1]
try the premium font 'Helvetica Neue'. It looks the same with/without antialiasing.