[R] ggplot2: font size mismatch for pdf output

[Michael Friendly]

Hi

In the following, the graph I see on the screen and the .png output
coincide.  However, in the .pdf file, the fonts seem to be scaled
fairly larger, resulting in the label for the top legend disappearing.
Is this an infelicity or bug, or is there something I've missed?

More generally, how do I control the size of fonts used in legends
and axis labels?

library(car)
library(ggplot2)

qp <-qplot (education , income , shape=type , size=women , colour=prestige ,
	 xlab="Education" , ylab="Income", data=Prestige)
    + scale_y_continuous(limits=c(NA, 20000))
qp + scale_size(to=c(1,8))

ggsave(file="prestige-ggplot.png", width=6, height=5)  # OK
ggsave(file="prestige-ggplot.pdf", width=6, height=5)  # fonts too large

-Michael