Not many of those 15 "full time Royals" actually work full time. The Countess of Wessex only works 3 days a week as she has small children, and she has only worked 3 days a week for a long time. The older generation (Duke of Kent, Gloucester's and Alexandra) don't work 5 days a week.
I agree with iluvebertie's statement regarding Catherine's small number. She may be pregnant but she can still do work. She isn't the first, nor is she the last, Royal wife to be pregnant and they have all worked harder than she has. Her workload could be reduced obviously, but she could still do more than a few appearances. Sophie worked more than Catherine has done when she was pregnant, even with James and she stopped working in October before his birth. She worked 2 days before she had Louise, but that was a different circumstance. Perhaps Catherine is in a precious condition...It as different in the 60's and 70's for pregnant Royals as pregnant women were not often "in public", but it's different nowadays.