There is a balance to genome size in terms of selection. If you have a smaller genome, mutations are more likely to have an effect, which means faster evolution (good if you are a pathogen for example). There are smaller energy requirements with replicating a smaller genome, and fewer atoms needed from the environment with shorter DNA. There is probably some optimal physical size to the nucleus, which gets larger as there is more DNA inside. Deletions can occur during cell division. A deletion bias is observed over a duplication bias:

https://en.wikipedia.org/wiki/Bacterial_genome#Deletional_bi...