Hi @carlosfherrera,
The behaviour you describe is the expected behaviour for GMT >= 6.5.0.
Please see similar posts for details:
- GMT forum: Multiple colorbars changes the annotation fontsize - #13 by JonathonLeonard
- GitHub PyGMT: Colorbar annotation size does not change · Issue #3041 · GenericMappingTools/pygmt · GitHub and following comments
- GitHub GMT: Fix incorrect font for colorbar unit by PaulWessel · Pull Request #6802 · GenericMappingTools/gmt · GitHub
Currently, users need to redo the scaling manually following the scaling factor \sqrt{colorbar\_width/15}, please see the note in the PyGMT documentation at pygmt.Figure.colorbar — PyGMT.