From the Max Planck Institute, we learn (April 21, 2011):
Despite being closely related to the lyre-leaved rock cress, the thale cress has a considerably smaller genomeIt would appear reasonable to assume that two closely related plant species would have similar genetic blueprints. However, scientists from the Max Planck Institute for Developmental Biology in Tübingen, working in cooperation with an international research team have now decoded, for the first time, the entire genome of the lyre-leaved rock cress (Arabidopsis lyrata), a close relative of the thale cress (Arabidopsis thaliana), the model plant used by geneticists. They discovered that the genome of the lyre-leaved rock cress is fifty percent bigger than that of the thale cress. Moreover, these changes arose over a very short period in evolutionary terms. This new high-quality genome analysis will provide a basis for further detailed comparative studies on the function, ecology and evolution of the plant genus Arabidopsis
Also,
A smaller genome appears to offer advantages during the natural selection of individuals.
Paper here free.