Regarding the font size of the colorbar
Up on GMT 6.5 for mode mode, an automatic scaling of the font size of the colorbar labels based on the width of the colorbar was introduced (please see #6802). In the dev docs there is a note (please see https://docs.generic-mapping-tools.org/dev/colorbar.html#description). There are several reports and comments on GitHub and the forum (e. g., GenericMappingTools/pygmt#3041 and Multiple colorbars changes the annotation fontsize) discussing how to handel this.
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