DPI means Dots Per Inch. DPI is used to measure the resolution of display and 
printing systems. A typical CRT screen provides 96 dpi, which provides 9,216 
dots per square inch (96x96). Flat panel displays from 110 to 200 dpi have also
been developed. The higher the resoultion(DPI), the sharper the resulting image.
Changing DPI's DOES NOT change the size of the resulting TIFF file.

Two articles are worth while reading in order to understand the terms used:

http://www.pptfaq.com/FAQ00075.htm

http://www.steves-digicams.com/knowledge-center/coming-to-terms-with-dpi-ppi-and-size.html

Download this package (to be used with the commandline):

p2t_faq.zip (1.4 M)

It contains:

300r600.pdf (The original PDF)
TEST_CMD_DPI.CMD (The command file to convert to test1 - test3.tif)
TEST_CMD_SIZE.CMD (The commad file to output test4.tif - reduce size)
test1.tif
test2.tif
test3.tif
test4.tif

You will see that modifying the dpi does not change the size of the resulting
file. Using -w -l changes the size of the file