Why do HK restaurants' wok fried scallops stay big? My pan fried ones at home shrink!
Regarding the scallops shrinking, it could simply be the difference in dry vs. wet scallops. Most grocery stores sell wet scallops. At times, dry scallops can be quite hard to find at retail. Wet scallops naturally shrink because of the loss of so much moisture when cooking.

Also, the higher the heat and longer the cook time, the more the moisture is lost.