I have the same harness for spike, just a different color (i bought the black one with the tan stars on it) he doesnt have any trouble. I dont know how much Gucci weighs, spike is about 4-5 pounds now and i have the smallest size, yours looks like it may be the size larger. Maybe the poster who mentioned it being loose might be right.
I havent really worked with spike alot on a lead yet (i know he's going to need it soon), but i know my shepherd really isnt allowed to pull me when hes collared. It looks like aggression to other dogs, so that when you are walking out it really works up strange dogs and increases likelihood of a confrontation or even an attack. I can see that hes just an adorable baby right now
I think its great that youre already working with him
when i walk my shepherd alone, alot of the time, i make him heel and then at every mail box i stop and he knows to sit at my left leg and then he has to sit until i tell him to go. I need to read up on how to train yorkies, you cant teach them by pulling on the lead.