Sounds like the idea behind this is rubber is a thermoplastic, so when it was moulded initially it was hot and was stretched out into shape, and held that way until cool. So it set at this size. But why you'd prestress it not sure (don't know v.much about rubber, possibly to make the clothes stronger in the stretched direction I spose so kids don't tear them when taking them off)
Sooo... I think heating it may allow the rubber to return to an unstretched state (sciencey blah, glass transition temp, molecules gaining enough energy to move out of a pseudo equilibrium to a lower energy stable equilibrium blah blah sciencey stuff).
Summary (soz am being a bit too keen I know!)
However I'd say really don't risk it unless you're willing to potentially destroy the outfit - since you've got no idea how much internal stress the rubber is under at the moment. So it might shrink a lot more than you're hoping for, if indeed heating it up does have any effect - it could very well do nothing at all.