Mitochondria evolved from an ancestral alpha proteobacterium. However, present day mitochondrial DNAs (mtDNA) from plants and animals are very different. While animal mtDNAs are about 16.5 kb in size, plant mtDNAs range from 200 kb to 2,000 kb. Gene numbers are similar, but most of the additional mtDNA in plants is composed of large introns, repeats and non-coding regions. Animal mtDNA are also usually circular, whereas plant mtDNAs are a collection of linear DNAs, along with smaller circular and branched molecules (see here for a review).
What different selective pressures might have led to these two vastly different evolutionary outcomes?