SUMMARY https://kde.org/plasma-desktop seems to use a bigger font size (see header) than the rest of https://www.kde.org.
The size is the same for https://kde.org/plasma-desktop and most of the KDE websites (e.g https://kde.org/community/whatiskde/). The font-size is defined in https://invent.kde.org/websites/aether-sass/-/blob/master/css/includes/_typography.scss The problem is in the homepage. https://kde.org contains tons of hardcoded font-size and is in the process of being updated completely. See the current mockup at https://kde.carlschwan.eu.
the link to the mockup is https://kde.carlschwan.eu/index-new and not https://kde.carlschwan.eu/
This is now fixed :)