When planted in the bottle, champagne corks were just as cylindrical as the corks used in still wine bottles, only bigger. A normal-sized wine cork is 24 mm (about 0.94 in.) in diameter; it is compressed and inserted into an 18 mm (o.71 in.) bottle neck by a “corker” machine. Cork is quite compressible.
Champagne corks, on the other hand, are 31 mm (1.22 in.) in diameter and are squeezed into a 17.5 mm (0.69 in.) neck, with the top third of the cork sticking out as a “head” that can be grasped for opening. As soon as it is liberated from confinement, the bottom portion, which is soft and wet, expands back to its original diameter. Cork is also quite elastic.
You can observe cork’s compressibility and elasticity by soaking a used Champagne cork in water for a few days to soften it, whereupon it will expand back to its original cylindrical form along its whole length. It will also revert to its original shape if you soften it by simply microwaving it for a couple of minutes.
Caution: Don’t operate an empty or nearly empty microwave oven. Radiation that isn’t absorbed by food or water can bounce back into the wave generator, the magnetron, and damage it. Put a cup of water into the oven along with the cork.