It does not save a bitmap of the image to a postscript file, but
normal postscript commands. The eps file is relatively large
(at least compared to png) because it includes a large but fixed
amount of font data. Try adding another line to your plot or
otherwise changing the contents of the plot, and you'll see size
of the eps file change.
Thanks...Originally I opened up the PS file in a text editor and saw lines of
hex data - I assumed it was a raster version of the plot. Now I see that hex
data is describing the fonts, and there are standard vector descriptors
at the end of the file.