I had this same experience sort of. We got our first batch of chicks at Easter from the local feed store. A friend of ours had gotten some the week before. Two weeks ago, my DH brought home all the roosters from our friend's batch as he didn't have the heart to retire them to freezer camp. I couldn't believe how scrawny those birds were! He feeds the same wayl. Cracked corn and lets them free range all day (but he really doesn't have the space for this). He came by our place last weekend and was amazed at how much weight they had put on in a one-week period. I have grower out for my birds continuously.
Yes, proper nutrition makes a world of difference.