Honestly, it sounds like you are feeding them enough. We've got standard feeders, like you have, and then the girls get some of their feeder scattered around the yard. Anything that they don't eat the local sparrows clean up. Combine it with some scraps, which should be cleaned up the next day if they haven't been eaten (or best before you put them to bed). Depending on the size, age and breed of the chicken will affect how much it eats and is difficult to provide an accurate amount. Additionally, feeders (bigger ones) that the chicken steps onto a plate, which lifts up the lid and then they can eat, is an amazing rat/mice proof feeder that can hold lots of seed, depending on the size you get. They are a bit pricy, so its optional but great if you want something rat proof because you'd need a very big rat to get the lid to open up. Ensure they have scraps to forage on plus their pellets. Every so often pick up your chickens (not just to check if they have lice, which should be a regular checkup by you) and feel them. If they feel very thin, increase their food and add in some high carb or protein extras (cooked pasta or some other types, check online for more) to make them put on but avoid fatty food. If they feel fairly plump, you know you're doing fine. They exercise a lot and burn a lot and I haven't seen any overweight chickens, yet.
