A girl who only has big titties because she herself is very small. Singular: Titmouse

A pubic hair piece. A toupee for the pubic area/genitals. In the 1700’s when mercury was used to treat sexually transmitted diseases (Gonorrhea or Syphilis) one of the side effects was the loss of pubic hair. To disguise this condition, that was not cured by mercury, a Merkin was employed.

Small, invisible rodent that leads what one imagines is an idyllic existence in the ample cleavage of beautiful ladies.

Neither a tit, nor a mouse. It's a bird believe it or not.

A titmouse is a very small insectivorous bird that come from the family Paridae and are only found in the northen hemisphere and Africa.
 
The name titmouse is attested from the 14th century, composed of the Old English name for the bird, mase (Proto-Germanic *maison) and tit, denoting something small. The spelling was influenced by mouse in the 16th century.

a short, petite woman with very large breasts (noun)

Small Northamerican song bird.