The use of various cex parameters in plot() is poorly documented and confusing, because it is inconsistent:
* cex.lab for scaling the labels is not used by plot() at all (AFAIK)
* cex.axis is used to scale the labels if a factor is plot()-ed with values
* cex.names is used to scale the labels if only a factor is plot()-ed
If you have long labels the needed use for cex.axis also makes the y-axis text small.
IMHO the perfect solution would be to use cex.lab whenever a label is output.
At least the documentation should clearly point out what is an "axis", a "name", and a "label".