Both plants and animals cells have vacuoles. plants however, have a
very large central vacuole, while animal cells vacuoles are quite
small. why is that?

The online Hutchkinson Encylopedia describes some of the differences
between animal and plant cells at
http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0015140.html
:

"Plant and animal cells share many structures, such as ribosomes,
mitochondria, and chromosomes, but they also have notable differences.
 Plant cells have chloroplasts, a large vacuole, and a cellulose cell
wall.  Animal cells do not have a rigid cell wall but have an outside
cell membrane only.

In biology, [a vaculoe is] a fluid-filled, membrane-bound cavity
inside a cell. It may be a reservoir for fluids that the cell will
secrete to the outside, or may be filled with excretory products or
essential nutrients that the cell needs to store.

Plant cells usually have a large central vacuole containing call sap
(sugar and salts in solution) which serves both as a store of food and
as a key factor in storing water and in maintaining turgor.  Absorbing
more water to make a bigger vacuole adds bulk to the plant.  This
expansion of cells is very important in plant growth.  In amoebae
(single-celled animals), vacuoles are the sites of digestion of
engulfed food particles.  Animal cells may only have small vacuoles,
which are usually called vesicles."

Also, Dr. Kent Simmons of the University of Winnipeg has published a
lecture note at http://io.uwinnipeg.ca/~simmons/cm1503/vacuoles.htm
that is useful in answering your question:

"Vacuoles

The term means "empty space".  But in the cell they are many membrane
bound sacs with little or no inner structure.  Plant cells have very
large distinct vacuoles.  In fact this organelles often dominates the
inside if the plant cell crowding all other organelles toward the cell
wall.  The membrane surrounding the plant cell vacuole is called the
tonoplast.  This a very active, dynamic membrane.

Plant cells use their vacuoles for transport and storing nutrients,
metabolites, and waste products.  In a sense, the vacuole can be
regarded as equivalent to the extra cellular space of animals.  The
simple space-filling function of the vacuole is of great importance to
plants, which capture energy from the sun rather than move to capture
food.  The mechanical stability provided by the combination of a cell
wall and turgor pressure allows plant cells to grow to a relatively
large size, so they generally occupy a much larger volume than animal
cells."

To sum things up:
Plant and animal vacuoles are similar because both can be used to
store or excrete substances.  The presence of large vacuoles in some
plant cells are caused by the plant having different needs than the
animal cell:

- a multicellular plant may need cells with a large vacuoles to support its
  structure, while an animal might use an exo- or endoskeleton for this purpose
- animal cells have a larger extracellular fluid volume where excess
  water (and solubles) can be stored
- animals grow almost exclusively by multiplying their cells, while plant growth
  also depends on cell growth

I hope this answers your question.  If not, please request an answer
clarification before you proceed to rate it.