If they’re not on layer feed now, I’d just cut back portions. I don’t think there’s a specific weight-loss feed.
Similarly, back off on the portions of veg/ fruit/ etc. Skip the scratch entirely.
Use the 15-minute rule: offer food for 15 minutes and then take up whatever hasn’t been eaten yet. You’ll probably get a great deal of Drama from them about this, but they’ll learn.
Since they can’t be out foraging a lot, consider adding some interest in their run. Branches of differing heights, upturned buckets to hop on and off of, piles of dried leaves to scratch through - anything that might get them moving a bit more.