Occasionally in evolution, especially in single-celled organisms, whole genome duplication events may occur. In these events, one descendant will have two complete copies of their entire genome, which can lead to rapid speciation, as the duplicated genes can acquire new functions.
One such event occurred in the evolution of single-celled yeast. The lineage leading to the baker's yeast S. cerevisiae experienced a genome duplication event. Curiously, if we look at closely related species whose ancestors did not undergo a whole genome duplication, we find that their genome sizes are fairly similar. For example, the yeast K. lactis is descended from a pre-duplication species and has a genome size of 10.6 MB, while S. cerevisiae has a genome size of 12.2 MB (MB refers to megabases or a million base pairs).
What is the MOST likely reason that the genome sizes in the post-duplication species are not double the size of those who did not undergo a duplication?