Gimp 3 alpha is pretty crazy, as GTK2 was very nice and usable, but already with GTK3 everything got huge, so now the buttons dont fit as well anymore.
I reported that issue to gnome a while ago: https://gitlab.gnome.org/GNOME/gimp/-/issues/9907
Hopefully they will fix it one day because it means I wont be using gimp 3 otherwise.