prayer book - Font size variation within siddurim

People might write this off as a matter of typeface and nothing more, but why do so many siddurim vary the font size within the tefillos? I used to think that they made the more important tefillos larger and the less important tefillos* smaller but in many siddurim there seems to be no order at all, they just change the font size randomly whenever they want. Is there a reason for the varying font size? Perhaps the publisher is trying to convey that certain portions of davening are more important than others?

^{*What even is a more important/less important tefillah? They're all important. I guess what I mean is that they make Baruch She'amar, Yishtabach, Shema, etc. large and things like various perakim from Tehillim small.}