That slider only arrived in Ubuntu very recently and I'm afraid it's not fully hooked up yet (when it is shortly it'll control the interface scaling & hopefully the gtk one too).
For now, take a look at: gsettings get org.gnome.desktop.interface scaling-factor - if this is '0' then the scaling factor is being autodetected. You can override it with e.g. gsettings set org.gnome.desktop.interface scaling-factor 1.
That slider only arrived in Ubuntu very recently and I'm afraid it's not fully hooked up yet (when it is shortly it'll control the interface scaling & hopefully the gtk one too).
For now, take a look at: gsettings get org.gnome. desktop. interface scaling-factor - if this is '0' then the scaling factor is being autodetected. You can override it with e.g. gsettings set org.gnome. desktop. interface scaling-factor 1.
gtk can only scale by integers...