In using the text tool in KolourPaint and Gimp, the font that I type in is way smaller than it should be. The image is 1377 x 2448. Dot for dot is checked under View.
And View is Zoom 25%.
In this pic the font is in the upper left hand corner.
The font size is the font size in the final image, before the zoom factor is applied. So at 25% zoom a 18px font is rendered as 4.5 pixels high (and even less so in practice form common characters).