Created attachment 157121 [details]
Screeshots of affected apps at different font sizes

I feel like someone must already have reported this, but I couldn't find it.

Setup:
OpenSuse Tumbleweed, Plasma 5.27.2, single 1080p screen, Intel integrated graphics, Wayland session, scaling at 100%.

Summary: 
A variety of applications show extremely small fonts and sometimes UI by default and do not respect font size, either chosen through the font size selector or through font dpi forcing. These apps include:
- All flatpaks (unresponsive to all font size changes)
- Firefox, Thunderbird, Chromium browsers, GIMP, Inkscape, Lutris and VLC (responsive, but starting at such a small size that by the time they are readable the rest of the OS has gigantic fonts)

STEPS TO REPRODUCE
1. Enter a Wayland session with default 100% scaling
2. Open one of the aforementioned applications, Thunderbird being the most blatant due to its condensed UI
3. Compare font sizes with Dolphin or System Settings. Try changing font sizes in settings.

OBSERVED RESULT
Some applications have extremely small fonts, that are either unresponsive to font size settings or increase but remain much smaller than the rest of the system fonts.

EXPECTED RESULT
All applications respect the general font size settings, without the need for increasing global scaling (which does make these apps legible, but makes the rest enormous)

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
The attached screenshots include one flatpak app (Flatseal), Thunderbird (non-flatpak, very small fonts) and the Plasma Settings app for comparison. Please note: the Thunderbird text in the screenshot responds to changes in the "general" font category and to none of the others, so it should be compared with that.

A temporary and very patchy solution for me was adding GDK_DPI_SCALE=1.25 variable in the .desktop file for each the concerned apps, except QT_SCALE_FACTOR=1.25 for VLC.  For Electron apps running them with --force-device-scale-factor=1.25 seems to work, and for Chromium browsers --force-device-scale-factor=1.25 helps. Flatpaks are unresponsive and I couldn't even find a hacky solution for those.