http://www.w3.org/TR/REC-CSS2/fonts.html#font-size-props

font-size-adjust would appear to help : quoting from the above page

In bicameral scripts, the subjective apparent size and legibility of a font are
less dependent on their 'font-size' value than on the value of their 'x-height',
or, more usefully, on the ratio of these two values, called the aspect value
(font size divided by x-height). The higher the aspect value, the more likely it
is that a font at smaller sizes will be legible. Inversely, faces with a lower
aspect value will become illegible more rapidly below a given threshold size
than faces with a higher aspect value. Straightforward font substitution that
relies on font size alone may lead to illegible characters.

For example, the popular font Verdana has an aspect value of 0.58; when
Verdana's font size 100 units, its x-height is 58 units. For comparison, Times
New Roman has an aspect value of 0.46. Verdana will therefore tend to remain
legible at smaller sizes than Times New Roman. Conversely, Verdana will often
look 'too big' if substituted for Times New Roman at a chosen size.

The problem is whether that's supported by browsers... according to
w3schools.com, it's in the CSS2 spec but currently unsupported
(http://www.w3schools.com/css/css_reference.asp#font)

-Simon