Comment # 12 on bug 1167341 from Fabian Vogt

The normal user has "force font DPI" set to 110 (which is a scale of
110/96~=1.14) and the scale factor set to 1.4. Result is a font which is
smaller than the scale factor.

The root user does not have scaling configured, so for the font size it uses
the physical monitor DPI for calculating the default, so either 144 or 192dpi.
Additionally, Plasma force disables scaling, so the font is big but controls
are small.

I assume the 110dpi is the value causing this mismatch, you should force the
font DPI to 134 to match the 1.4 scaling or disable forcing altogether.