This is because all you other ticks are mathtext
except the first one, the easiest fix is to use mathtext
for the first one as well, such as:
xtck = ['$60m$'.......
Now they will be of the same style.
'60m' should appear somewhat larger in the end, as the others are fractions.
If you are trying to force the 60m
to appear as the same size as the others, change it manually by adding this line in the end:
ax1.xaxis.get_ticklabels()[0].set_fontsize(10) #10/16 is almost the Golden ratio