Would it be possible for flossing to help shrink a cavity?

Absolutely not: A carious lesion (cavity) that can be detected interproximally (where you are flossing) is already big enough to require a filling. Have the cavity restored, and then floss to help prevent further decay on all of your teeth.

See a dentist: Once tooth structure is lost, the only way to fix the cavity is by having a dentist drill the decay and replace it with restoration. Flossing helps remove plaque and food particles stuck between your teeth. See a dentist.