When rendeing heading elements we are using Boostrap helper classes .h[1-6] to reduce the size (<h1 class="h2">, <h2 class="h3">, ...). Although it can be useful in some specific cases, it shouldn't be done as a rule in our basic layout.
Some code examples:
https://github.com/moodle/moodle/blob/main/theme/boost/classes/output/core_renderer.php#L195
https://github.com/moodle/moodle/blob/main/course/format/templates/local/content/section/header.mustache#L40

It would be a cleaner approach to adjust those values directly in the BS variables adding them in "default.scss"

$h1-font-size:                $font-size-base * 2 !default;

$h2-font-size:                $font-size-base * 1.75 !default;

$h3-font-size:                $font-size-base * 1.5 !default;

$h4-font-size:                $font-size-base * 1.25 !default;

$h5-font-size:                $font-size-base !default;