Do you feel the ribs on your yorkie when he stands
?
You should not have a problem with feeling them but you should not see them easily (should not stand out) , if you have no problem with feeling the ribs then he's fine
If you don't feel the ribs easily then he's too fat ! then you have to put him on food with less fat, less calories and more protein, and also keep exercising him well.
You should see a waist on the dog
If he doesn't have any waist then he's too fat.