[R] Issue increasing DPI on a png output of a plot

[Justin Fincher]

Howdy,
  I have created a set of plots, but I wish to increase the dpi to 300
(instead of the default 72).  From the documentation, I thought that
the "res" parameter to png should accomplish this, but it appears to
greatly alter the appearance of my plot. (plot area becomes smaller,
plot lines become thicker, etc.)  It is my understanding that
increasing the dpi should not change the look of the plot, just the
quality.  Any help in recreating my plots in 300 dpi would be greatly
appreciated.

- Fincher