I believe that they need CALCIUM asap...The easiest thing to do is to get oyster shell from your feed/pet store. I put in in with their feed, which by the way I use layer 'crumbles', my girls liked it much better than pellets (they have no teeth). I also buy and mix in their feed dried alfalfa. Don't mix the grit in their feed, just put a handful on the ground somewhere, they will eat when they need to.
Also, yes too much corn will make them fat... Maybe don't just give then corn, get them Scratch Grains and feed as a treat. It has other grains in it in addition to cracked corn... but it's a treat...
I'm super big on feeding my girls (I have 4 hens) food I make for them. A great calcium source is greek yogurt, yup, yogurt. My girls get it once a week, about 1/2 cup mixed in with oatmeal or rice or pasta, along with cut up grapes or raisins, banana and apple sauce. But you can just feed them the yogurt plain. Only give them plain yogurt not the crap with tons of sugar and franken-fruit in it. Yogurt is a great source of calcium with the bonus benefit of good bacteria to help keep the crop and digestive system all happy. I will spare you my tales of what else I feed them..
But I think your girls need calcium, asap.. Best of luck, Helen