It's because you have the Dynamics on, (thanks to your screenshot with the right tools/tabs on it)
and this basic dynamic change the size of the brush with the speed in which you move the mouse/pen as well as fading and changing opacity and so depending the pressure you're applying on your pen while drawing
One quick solution is to click on that big button (arrow down with 3 dots above) to open a drop-down menu and select Dynamics off
more about dynamic brushes https://docs.gimp.org/2.10/en/gimp-tool-dynamics.html (that's already a good reading if you use a mouse, it's even more important if you use a tablet)
and this basic dynamic change the size of the brush with the speed in which you move the mouse/pen as well as fading and changing opacity and so depending the pressure you're applying on your pen while drawing
One quick solution is to click on that big button (arrow down with 3 dots above) to open a drop-down menu and select Dynamics off
more about dynamic brushes https://docs.gimp.org/2.10/en/gimp-tool-dynamics.html (that's already a good reading if you use a mouse, it's even more important if you use a tablet)