A medium sized creature is in all of the squares of his mount (no matter the size).
However there are exceptions to this rule.
If the medium sized creature has an aura, you declare an origin for that aura and it emanates form that square (rather than the entirety of the mount)
If a close burst power is used, you pick an origin square for the power (rather than using the entire area of the mount.)
For attacks you are considered to occupy the entire area that your mount does. Attacks that target all creatures/enemies can target both of you, attacks that target a single creature must choose which of the mount and the rider to attack.