While calculating the conductor impedance the parameters that we consider are the conductor width, conductor thickness, frequency & substrate parameters. For achieving the particular impedance based on the material properties we are fixing the trace width. What about the current which depends on trace width ?
This might help you out: what is the current limit through a trace?
(Would've included this as a comment, but dont have comment priviledges yet.
For high-power rf circuits, you do want to have a wider trace to reduce losses and self-heating of the trace.
Luckily, the characteristic impedance depends mainly on the ratio \$w/h\$, where w is the width of the trace and h is the height of the trace above the ground plane. So you can make your trace wider and keep your characteristic impedance constant by increasing h. (Taking care not to increase the dimensions so much that your transmission line becomes multi-modal at your operating frequency)
This could mean changing your stack-up or making cut-outs in layers directly below the trace so that a lower layer will be used as the return path for the rf signals.
You might also even use a different substrate, such as ceramic, rather than fiberglass, with better thermal conductivity. This won't reduce the rf losses, but it will reduce the temperature rise of the trace, allowing more power loss before thermal limits are reached.