Thanks for the extended and information rich answer!

> in the latest version control, small caps are greatly expanded relative to the 2012 release

Is there any "built" version of the fonts that is recent enough to try?

> the URW fonts on which the Latin faces are based were meant to resemble the Adobe Times-Helvetica-Courier set, not on the Microsoft faces

Obviously... My mistake: I wrote MS in place of Adobe in my report...

> I also found this discussion:

https://graphicdesign.stackexchange.com/questions/125452/how-and-when-should-petite-caps-be-used

While I am not by any means an expert, I have always got the feeling that the matter is about the fact that there are different uses for small caps which may involve having them either in running text together with small letters or in isolated environments with no close by small letters (e.g., headings). For the first case, it is nice to have the small caps as tall as the small text because they mix more harmonically in this way. However, for the second case, the visual effect is generally more pleasing if the small caps are taller, at least for fonts with a little x-height, because the intention is more similar to that of giving emphasis in an all-caps like way (think of ringing with capitalis monumentalis, inscriptional capitals, elegant capitals), while still wanting to maintain non-capitals recognizable from the real capitals. Distinguishing small and petite caps satisfy both these needs.

Apart from any theoretical discussion, which is quite interesting, there is a real practical problem, though. This is when you need to adhere to some institutional, departmental or editorial style guide which prescribes typesetting rules based on  the appearance of some commercial font (typically Adobe or Monotype Times or Times New Roman). E.g., if you are invited to do headings in Adobe Times Small Caps, you cannot really use something visually diverging from it.

The Small Caps provided in GNU Free Serif have a visual aspect that is very different from that of commercial Times Roman or Times New Roman counterparts.

To the best of my understanding one of the values of Free Serif is to be an almost seamless replacement of commercial Times Roman offers. The rooting of GNU Free Serif in URW++ Nimbus Roman that is metrically compatible with Times Roman seems to support this point. If this is the case, divergence on Small Caps is an issue as it may hinder usage of Free Serif in place of commercial fonts when Small Caps are required and visual consistency is a must.

The issue is that GNU Free Serif has Small Caps letter with a very reduced height. In fact they are as tall as a small case "x". While this is a common choice, in fonts with relatively low x-height (such as Times Roman), small caps are generally somewhat larger than this. As a matter of fact, in Adobe/Microsoft/Monotype/Linotype offers the Small Caps height is larger than the x-height (probably 20-30% larger, about 70% the height of full capitals). For these cases, the x-height form is generally called Petite Caps preserving the Small Caps for the larger variant.

Note that the same issue is also present in TeX Gyre Termes and other free fonts. This basically leaves users without a Times Roman metrically compatible free font with visually compatible Small Caps:

- STIX Two is Times lookalike with proper height Small Caps, but is not metrically compatible with Times Roman (particularly in the bold variants);

- STIX is obsolete and does not have Small Caps;

- XITS does not have Small Caps;

- TeX Gyre Termes has too small Small Caps (in fact Petite Caps);

- Tinos does not have Small Caps;

- Thorndale does not have Small Caps;

- Tinos dows not have Small Caps;

- Linux Libertine and variants have too small Small Caps (again in fact Petite Caps).

Would be great if GNU Free Serif could move its current Small Caps offer under Petite Caps and provide Small Caps with a similar height as that in commercial counterparts.