[XeTeX] fontspec and scaling

[maxwell]

I have a question about how scaling is done in fontspec.

We produce some multi-script documents (grammars).  It's sometimes the 
case that for the non-Roman script, the glyphs at the normal point size 
seem (in comparison with the Roman script glyphs) small.  For example, 
we're using a Dhivehi (Thaana script) font which at any given point size 
looks quite a bit smaller than the Roman script.  In part, this 
difference is probably compensating for the fact that vowel get stacked 
above (or occasionally below) consonant characters, so the Thaana 
consonant and vowel glyphs are smaller to make the line height similar 
to what you'd get with a Roman font at the same point size.  Regardless, 
the Thaana is hard to read unless we make it somewhat larger; at the 
same time, I don't want to go to a larger Roman point size.  (Frankly, 
at my age *any* font is hard to read at a normal point size.  But I 
won't go there...)

The same thing happens with Arabic, particularly in the Nastaliq style.

In order to enlarge these Thaana glyphs into something readable at a 
given point size, I've used fontspec's "scaling" attribute, e.g.
    \newfontfamily\thaanafont[Scale=1.4,Script=thaa]{Mv Elaaf Normal}
Mv Elaaf Normal is an OpenType font.

One effect of this is that for any line where a Thaana word appears, the 
separation between that line and the following line increases.

Apart from this, is there any negative?  I'm not familiar with font 
technology, but I have heard that it's not the best method to simply 
magnify a font to be a larger size; rather, some things in glyphs may 
change in proportion as the glyphs get bigger.  (Or maybe I'm just 
making that up, I can't find a reference to it now.)

How does Fontspec do scaling?  Do I get the same typographic results by 
using Scale as I would if I simply specified a larger point size?

    Mike Maxwell