The study of genome size diversity is an ever-expanding field that is highly relevant in today's world of rapid and efficient DNA sequencing. Animal genome sizes range from 0.02 to 132.83 pg but the majority of animal genomes are small, with the most of these genome sizes being less than 5 pg. Animals with large genomes (> 10 pg) are scattered within some invertebrates, including the Platyhelminthes, crustaceans, and orthopterans, and also the vertebrates including the Actinopterygii, Chondrichthyes, and some amphibians. In this paper, we explore the connections between organismal phenotype, physiology, and ecology to genome size. We also discuss some of the molecular mechanisms of genome shrinkage and expansion obtained through comparative studies of species with full genome sequences and how this may apply to species with large genomes. As most animal species sequenced to date have been in the small range for genome size (especially invertebrates) due to sequencing costs and to difficulties associated with large genome assemblies, an understanding of the structural composition of large genomes is still lacking. Studies using next-generation sequencing are being attempted for the first time in animals with larger genomes. Such analyses using low genome coverage are providing a glimpse of the composition of repetitive elements in animals with more complex genomes. These future studies will allow a better understanding of factors leading to genomic obesity in animals.