Is there a bug in the annotations that creates excess margins when then annotations are not aligned with the plot edges?

A colleague was trying to rescale some plots for a publisher’s required sizes. For plots where the annotations are aligned with the edges of the plot, the PostScript files had the correct size in Illustrator. When the range was reduced and the annotations weren’t aligned, GMT added extra space to the x-axis labels.

For example, when the range of the x-axis is changed from 4.2 to 5 in these commands,
gmt psbasemap -R0/4.2/500/550 -JX5.2/3.5 -P -Bxa1f1 -Bya10f -BneWS >
gmt psbasemap -R0/5/500/550 -JX5.2/3.5 -P -Bxa1f1 -Bya10f -BneWS >
the width in Illustrator changes from 7.1317 cm to 6.4338 cm.

Is this a known bug?

I tried your example with gmt 6.0.0, 6.1.0 and 6.2.0.
Using the command:
gmt psconvert -A -Te -E72

to get the eps file, all were 183x124 in size, what is about 6.45583cm in width.

Could be a issue with Illustrator?