Final answer:
The IGF-1 gene is responsible for the diversity in breed size among dogs, as studies have shown its significant association with body size variations in different dog breeds.
Step-by-step explanation:
The gene that plays a significant role in determining the diversity in breed size among dogs is the IGF-1 gene. Insulin-like Growth Factor 1 (IGF-1) is a hormone with a molecular structure similar to insulin and plays an essential role in growth regulation during both foetal and postnatal life.
Studies on canine size diversity focus on the effects of single nucleotide polymorphisms (SNPs) in this gene, which are significantly associated with variations in body size across different dog breeds.