Only actively laying hens should be fed layer pellets. If you have birds of different ages, sexes, or not actively laying hens, feed a 'flock raiser' or 'all flock' feed. Supplement laying hens with oyster shells for calcium. Have feed, calcium, and grit available all day long; not just a couple times a day. You can give them treats occasionally and never more than 10% of their daily diet. Fruits, veggies, leafy greens, chicken scratch, grains, and meal worms are good treats.
The average chicken like yours are full grown by 6-8 months. They fill out a little bit over the next few months and that is it. They don't grow anymore after this.
For baby chicks, it really is best to feed chick starter/grower. Not sure if you were feeding them mainly small bird seed when they were little, that's not the best diet and could have been the reason for their size. If so, there is nothing that can be done about it, besides giving proper feed from now on.