plot.ts - change font size of labels & axis.labels

Question

When I plot a time series (f.e. cardox from astsa-package) I am not able to change the fontsize of labels and axis.labels:

library(astsa)
class(cardox)
plot(cardox)
plot(cardox,cex.axis=0.5,cex.lab=0.5,cex=0.5)

The plots look the same, the fontsize didn't change, despite the different cex-attributes.

What is the reason for this behaviour and how can I fix this?

Thanks!