I'm confused in one post you said you are trying to get them to lose weight and in another post you said they are underweight???:dunno
For overweight rabbits:
I believe you have holland lops. For show hollands 1/2 a cup is plenty. I feed just the 1/2 cup of plain pellets and timothy hay daily to my rabbits. You can also give them minerals in their water and supplements on the side to help improve condition, you should be able to purchase this from your local rabbit vendor at an ARBA show.
Underweight rabbits: I always have a measuring cup when I feed my rabbits. Are you 100% sure you are giving them 1/2 cup a day? You can increase the food amount if needed. Have these rabbits been heavier or ever in good condition? I know some rabbits genetically have bad condition where you can feel more spine, pin bones, and ribs. I have one rabbit that genetically has bad condition, she is used as only a showmanship rabbit. Condition is how to flesh is on the rabbit (thats how I take it at least) You want to make sure the diet is consistent and they are being fed at similar times. You can add supplements to their feed and to the water. This should help them. You can feed Calf manna (purchase at local feed store) and feed a teaspoon to them to help them gain some more weight. You can also used rolled oats to help aid in gaining weight. For pellets make sure you pellet has at least 16% protein, around 18% fiber, less than 3% fat and under .5% calcium. Because your rabbits are show rabbits and they are unaltered they have different feeding requirements so they need to be fed a diet to suit their needs. If the rabbits don't seem to be gaining condition but they are gaining weight it may be better to pet them out and look into getting rabbits with good condition. This happened to me with my lionheads.