I always assumed the shape of a given contrail was goverened by the wake-vortex pattern of the specific a/c type and its interaction with local atmospheric conditions.
Interestingly, yesterday was an exceptional day for contrails; some were visible from here stretching completely unbroken from a/c entering over Clacton routeing to HEMEL and routeing north and visible to overhead Pole Hill ! - Not sure of the track miles involved - but it must have been very cold and still up there.