To be honest in my experience you sometimes need to leave more like
3/4" - 1" with a synthetic cork if you expect it to plug the neck
without soon rising above the rim. *

I've never done this, so if there is someone with exerience with this,
please join in: I have heard of placing a stainless steel wire
alongside the cork during the insertion of the cork. This leaves a
small space for the air being compressed by the cork's insertion to
pass by the cork. Then, the wire is removed, and since the cork is
still just freshly inserted, it re-expands to take up the space the
wire held, creating a full seal. I would think this is more useful
with a synthetic cork than a regualr cork.

As Frederick says below, "Air is compressible, liquids are not." Air
is also expandable, which means that the removal of the cork when
you're ready to drink is a lot easier with a little air space between
the wine and the cork.

Rob