pointsize - what is wrong?

Hello!
When I am trying to produce plot with text with pointsize 30 or
greater R sets up a default pointsize of 12 on plot.
I need to set up a large pointsize to make large jpeg or
png files (with good resolution).
I can write text on plot with pointsizes 18,20,22, but i cant
write text with pointsizes 25,26 and greater and i can not
understand - why? Is there are any restrictions in R? Or what?

I am using R 1.7.0 on Win2k

Thanx.