Keep in mind that some dogs don't have a fully defined waist - they can have a "cobby" build.
I'm thinking that if your vet didn't mention it, it's probably okay... most vets are very alert to overweight pets. You could also post some pictures here to get our opinions if you like.
FYI, the correct term is "porky yorkie".
I walk Thor or take him to the park for at least 30 minutes every day, and I try for 60. I guess I'm lucky in that he gets antsy and barky after even a day or two of insufficient exercise... he keeps me on my toes.
On the weekends, I try to get one long afternoon activity with him, be it a hike, or a yorkie social.