Amy Teitel examines it:
It turns out that a completely prepackaged menu is more than twice as heavy as the bioregenerative alternative, even with a significant fraction of the foods freeze-dried. The prepackaged diet was also found to have lower nutritional value; the light and heat used in sterilization does have some negative effects on the foods. But having food ready to eat does shorten the time crew have to devote to meals, freeing up more time for the things that burn calories like space walks.
(Image: Shuttle food tray from NASA.)