Growth rates depend on genetics. My boys are full brothers from different litters and they had growth spurts their first year. They would stop growing and then have a spurt. Their last one was at 11 months, but that is a little later than the norm.
Your boy won't be chunky, just bigger.
My boys are both over the standard Yorkie 7 pounds.
For clothes, I would buy only necessities until he is a year old, knowing that he will likely outgrow everything you buy now.