Chin come in all sizes, from about 400 grams to over 1,000g for adult weight, so especially if they aren't related they could just be different size chins. If they are siblings then one could just be growing faster then the other, they can also grow at different rates, or have grow spurts at different times.
Meet our 3 newest chins, they were born fresh this morning. I have photos but they are too large to post here, I will post when I figure out how to compress the file more. If anyone can help that would be great!