Welcome to YT! We are glad to have you here. There are many here that can answer your questions you will have.
Here's a link to an average puppy growth chart.
Yorkshire Terrier Puppy Growth/Weight Chart
You can find their age on the left when they were 9 weeks old then go over to the right until you find what they weighed. If you go down that column it will show you the average weight gain until they are full grown.
You would be wise to switch to a higher quality food. They will definitely poop less and have better nutrition. There are many good quality foods out there, but they are
not sold at grocery stores or Wal-Mart. I feed mine Canidae, but there are many others that are good as well. Once you get a good food don't keep switching or you will create very finicky eaters. If they are hungry enough they will eat.
Hope this helps!