Yes, creatures smaller than Small smaller creatures take up fractions of a 5-foot space. This means that multiple creatures smaller than Small can occupy the same space, so long as the sum of their spaces doesn't exceed the 25 feet2 available in one 5-foot square.
Very small creatures take up less than 1 square of space. This means that more than one such creature can fit into a single square. A Tiny creature typically occupies a space only 2½ feet across, so four can fit into a single square. Twenty-five Diminutive creatures or 100 Fine creatures can fit into a single square.
Those sizes of creature can also always enter the spaces of others (though it provokes an attack of opportunity):
A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.
This means that your familiar and wizard example works — the Tiny familiar can sit on the Medium wizard's shoulder just fine.
All creatures can also move through, but not stay in squares occupied by another creature that is three Sizes different:
Any creature can move through a square occupied by a creature three size categories larger than it is.
A big creature can move through a square occupied by a creature three size categories smaller than it is.
This means that the Medium wizard can't occupy the same space as a Huge summoned creature, and can't even move through its space (Huge is only two Sizes larger than Medium). A Medium wizard still can't occupy the same space as a Gargantuan summon by moving into its space and then stopping, but the wizard could move through its space.
Does that seem inconsistent, since a Tiny creature can enter a Medium creature's space, but a Medium creature can't enter a Huge creature's space? Sure, and there are times when a DM might overrule that, but it's the default set by the rules.
An exception would be climbing on the larger creature (up to DM adjudication), or using a larger creature as a mount, which the Medium wizard could do with a Huge summoned creature. (Again with DM adjudication, though this is relatively uncontroversial. Though if it's not a type that is appropriate as a mount, the wizard would suffer the usual −5 penalty to Ride checks.) This is occupying the same space in a different way that you're meaning though, which seems to be sharing the space, both independently standing, but it's an option.