Plastic is a polymer, like cellulose. It's well known that microbes have evolved some enzymes to breakdown biopolymers - cellulase, which is present in rumen fluid. It is also known that pseudomonas has enzymes that are capable of breaking down hydrocarbon chains in oil - since that is the precursor for plastic polymers it should be no surprise that there are pseudomonas strains capable of breaking down plastic polymers. This paper claims that there is such a strain that is dominant in this rumen fluid that does so through a hydrolysis reaction (likely using an enzyme produced in that strain as a catalyst). Hydrolysis cleaves terephthalic acid in this process - a monomer that contains an aromatic ring. So essentially the final size of the polymers depends on the placement of that unit in that particular polymer. You need several enzymes working in sequence to degrade a complex polymer fully, but it is possible.