You need to state your claim.
YOU have said retroviruses make the genome bigger.
Retro virus. You need to explain your claim.
Oh okay my claim was that the genome size can increase through DNA insertions from mechanisms like retroviruses.
I was trying to get you to expand on your claim that that is wrong, but I'll go first if you aren't familiar with retroviruses:
Retroviruses are viruses that insert their DNA into ours, then like regular viruses, highjack our cellular machinery to replicate themselves. Often they lay dormant for a long time, and often they entirely fail to activate or are harmless, and the host cell survives with it's new section of inserted endogenous DNA.
If this happens in a somatic cell, this vanishes within the generation, but if it happens in a sex cell (sperm, egg, recently fertilized egg) it can be passed on to all the cells of the body and down through the generations as a permanent addition to the genome.
Unrelated to how this lengthens the genome, it's also a very powerful evidence for common ancestry. Some endogenous retrovirus codes (ERVs) insert at specific sites in the genome, but some are known to insert in a random spot, e.g. after any 'gttac', of which there are an astronomical amount. When one of these randomly placed ERV codes is in the same location in all the members of two populations, it amounts to very strong evidence that the ERV infection happened in a common ancestor to both population.
It is astronomically unlikely for any two organisms to share the same randomly placing ERV code in the same spot in the genome, and it not be the result of inheritance- and all life shares some ERV codes, the most ancient ones. Others are shared by groups of families of animals, painting the family tree in extraordinary detail.