One point concerning Metafont and Computer Modern (and TeX): A key overall goal of the whole kit and caboodle is that a document written in TeX and using the default fonts would produce exactly the same output on all capable computers and output devices, indefinitely into the future, with the same line breaks, page breaks, character shapes, etc. And in fact TeX files from over half a century ago still produce pixel-perfect results.
This requirement meant that the default fonts had to be portable between printers and typesetters with various resolutions (and other optical filtering attributes). There was simply no other way at the time to accomplish this than using a tool like Metafont to author a home-grown font family like Computer Modern (and even then it was a bit of a hope and a prayer).
And don't even get me started on how 5pt super-super-scripts aren't just 10pt characters scaled down by half, contrary to how post-hot-lead typesetters operated, and not fully addressed commercially until decades later.
Metafont users can specify point size, boldness, slant, and whatever other parameters they choose (plus device-specific info like resolution) that can then be used in the simultaneous equations that determine the coordinates of critical points and shapes of pens used to draw the characters. So, for the smaller-font issue, the width of the pen may be a non-linear function of the font point size; ditto for the x-height, etc. In Metafont, it's the responsibility of the font designer / coder to handle it all; there's no automated algorithmic monkeying with the pixels other than what you write. This all becomes much clearer with a perusal of The METAFONTbook.
https://tug.org/interviews/fuchs.html
a useful starting point.
