During polymerisation (the process in which 'monomers' crosslink chemical bonds to become 'polymers') most chemical substances physically contract in volume. This effect can cause problems in plastic moulding processes.
Over the years, however, a number of chemical monomers have been discovered which expand as they polymerise (typically by around 10-15%) - they are therefore added to the plastic moulding mixes so as to stabilise the moulding process.
The expanding monomers are mainly based on 'cyclic' compounds (atomically structured as a ring) and it has been found that the larger the ring, the larger the expansion. The complex atomic mechanisms which drive the expansion are not known.
See Wikipedia