Loss on narrow microstrip line & on wide microstrip line

Assume 2 microstrip line are both 50 Ohm impedance but the first trace is
narrow and the second trace is wide by different PCB stack.

Which one get bigger loss ?
The narrow trace get bigger loss or they get the same loss?
(Both of them are 50 Ohm impedance)

Thank you.

generally speaking,the narrow one 'lossing is low!

you can see a structure of microstrip to waveguide, wideband insert loss is about 1 dB but narrow band could be more than less than 0.3 dB!

Loss of μstrip depend by the conductor loss and dielectric loss.
In most cases the conductor loss dominate the dielectric loss.
As the current is more distribuited as the metal loss, of course, decrease.
So larger is the W as the dissipative loss is low.

... but... as large is W as large is the fringing capacitance, so... pay attention to the mismatch due to the end effects.

dielectric loss depends on substrat conductivity, so it will be different for different substrate.
discontinuity of microstrip line such as bending, open end also make loss.

Added after 6 seconds:

dielectric loss depends on substrat conductivity, so it will be different for different substrate.
discontinuity of microstrip line such as bending, open end also make loss.

AWR's free Txline program
hxxp://www.mwoffice.com/products/txline.html
may show you actual losses for different substrates and conductors. Perhaps experiencing some examples may show you the right directions.

Both DC resistance per unit length, and the skin effect loss per unit length, will be greater in the narrow trace. Those two factors will cause the signal loss in the narrow trace to be greater than those in the wide trace at all frequencies.

The dielectric loss will be approximately equal in the two at any given frequency, assuming the same dielectric is present above and below both traces.