The question is actually manifold. According to the Divrei Chaim (Shu”T) each shevet had its own nusach of tefiloh that was specific to that shevet. Over the years of golus, these nuschaos were lost to us, and we adapted to a prevailing nusach according to region where we lived. There were nuschaos (with some variations) for Edus Hamizrach – Sefardim. In Ashkenaz, there were also several nuschaos. One of the leaders in compiling nusach was Rav Wolf Heidenheim, a talmid of Rav Nosson Adler, and chaver of the Chasam Sofer. He published his siddur in Roedelheim, and it went through about 8 revisions during his lifetime, with changes and improvements each revision. The reason for some of these revisions was a sefer published by a left leaning maskil in which there were fabricated nuschaos that were becoming accepted by an innocent population who were unaware of these forgeries. Slowly, several of these were excluded from the Roedelheim siddur. (Some of these forged nuschaos still exist.)
This siddur became the accepted base for all nusach Ashkenaz. Today’s published Nusach Sfard (not Edus Hamizrach) is a publisher driven composite of Ashkenaz, plus aspects of Nusach Ari that were accepted among Chassidim. This Nusach Sfard has become user friendly, and includes many bracketed words that some say, as per their minhag, while others do not.
While some publishers (notably Artscroll, Avodas Hashem – Belz) choose to have everything in a single size font, others alter the sizes of font to make the siddur more comfortable to use and easier on the eye. And this may lead to an implication that certain paragraphs are more important, which is incorrect.
Nusach is actually a fascinating subject, and it is sometimes easy to discover that the intended tefiloh is different according to a different nusach. Far superior to the subject of nusach, or the publication of the siddur, is the kavannah and emotional/intellectual investment that comprise the main aspect of tefiloh. We can all use some chizuk and growth here. May we all be zocheh to daven with full emotional involvement.