Others may have more to add but my understanding of the "tuck up" is it's more dependent on the anatomical structure of the dog (relating to sternum length/bone length) rather than a dog being over or underweight.
Weight or fat is usually related to and assessed by
1. The rib test (feel or see)
and
2. The waist indent viewing from above (like in your photo).
So I don't think you can create a higher tuckup by dieting when Bobby is not carrying any excess fat there. Also, my understanding is that labs being assessed as conformation labs are not to have such a tuck-up.