How often and how long is it shrinking down in size for? It is normal for anenomes to expand and deflate to remove wastes and replace stale water in thier systems for fresh water. It is a normal life process that should only take a few hours. The concern comes in when the anenome is deflated for extended periods of time or is doing so repeatedly and never fully expanding.
The first action to take would be a water parameters check, followed by a water change of about 20% then double check your lighting. Insufficeint lighting, too much water flow, poor water quality and overfeeding can also contribute to deflating.
Also, it appears it has moved in your photos, how often is it moving? Constant movement is a definate sign of stress where the conditions of the anenome are not being met and it moves to find a suitable spot.