ANSWER
You should not have a problem with composite decking shrinking.
Composite boards are made of 50 percent  plastic and wood fiber by
products.  You shouldn't have any shrinks problem.

You should however,  make allowances  for expansion and contraction of
composite and plastic boards. It is important to note that all composite and
plastic-based products will experience some degree of expansion and
contraction with changes in temperature. Longer boards expand more than
shorter boards

When screwing the deck you should pre-drill before you screw. Also you
should use screws that is meant  for composite decking. One of them is
called   
TrapEase screw                                                by Master Fastener uses
a built-in secondary thread system to pull excess material down as the
screw burrows in. This prevents mushrooms from forming.  Also you can lay
down some marine-grade polyurethane adhesive  between the joist and the
deck boards.  After the adhesive has dried then you can screw into the deck.
That is what This Old House general contractor Tom Silva does.
Composite Decking
                              QUESTION
I am building a deck using composite material  for the floor
boards. How do I keep them from moving? If I drill them
the composite material will shrink and the deck will warp.
Installing Composite Deck Boards

  • Composites are not no-maintenance decks; they're low-maintenance
    decks.

  • Composites carry 10-year warranties: better than wood (which has
    none), but less than the 50-year offers for solid plastic decking.

  • The fiber content  in composite decking improves plastic’s stiffness,
    reduces its tendency to move as temperatures change, and roughens
    its texture for improved skid resistance. It also makes the material
    easier to cut and install than all-plastic decking. Composites do not
    require a finish of any kind, making them easier to maintain than solid
    wood.

  • There are some drawbacks to composite lumber though. Because of
    the natural fibers in the material, it absorbs water (although not nearly
    as much as solid wood decking), making it vulnerable to decay. It can
    stain and discolor from food spills or grill grease. Plus, textures in the
    wood tend to hold dirt, which can be a good environment for surface
    mold and mildew. As a result, composite decking needs to be cleaned
    periodically with a scrub brush and an off-the-shelf deck cleaner or a
    detergent that contains bleach.
Information Courtesy of This Old House
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Plastic vs Composite decking
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