Hi,
there are not difference betwwen 10 kg and 10000 kg load cell.
First if your target is 1 kg accuracy, this is a 0.01 %. You must use a very precision component in your schem. I am not using the AD7705 or similar directly, but may be for this accuracy this not very good idea. What type of opamp you use in discrete decison. You must use a very precision opamp-s, the best way is to use a chopres, like an ICL 7650. What is you Vref source? Hw you made sample, if you give only one sample, this is more that normal. You must get many sample, and after then get the averrage, the some simple is to get 32 or 64 or 128 samples, add to one variable, and after that shift this variable to right with 5,6 or 7 bits. The sigma delta ADC has a some stranges, i cag suggest toread carrefully the data sheat. To build similar application i use the ICL7650 and TC500. If this is a truck scale, then you must has a algorithm to reject the error from movement and vibration. How you supply the cell, you hav to diferent way, one is to supply directly from Vcc, and use to same methode to get the Vref, second is more compliced.