You might also want to compare the quality of different foods. I'm sorry to say that iams is not the best food. Also, some higher quality foods actually require smaller portions.
you can compare foods at
http://www.dogfoodanalysis.com/dog_f...iews/index.php
the 6 star are probably too much for a yorkie - so look at 4 & 5 star. You can see how iams rates here