From the link that I posted on page 1.
Egg production requires energy, energy that beyond normal metabolic functions would typically go towards growth. Males are not as greatly affected by spawning as sperm production requires less energy than egg production.
Then factor in that spawning females often feed less, some far less, than when not spawning (as can males) and the slower overall growth part of the equation should become rather obvious. Less energy intake via feed, more energy going to egg production vs growth = slower overall growth.