1. 100 kg is a heavy weight.
2. 1,000 dollars is a great sum of money for the boy.
3. 1,000 dollars is a big money for him.
4. 1,000 meters is a long distance to get.
Are all the expressions grammatical?
1 is fine.
2 is fine.
3 should be -- 1,000 dollars is a large amount of money for him.
4 should be -- 1,000 meters is a long distance to walk.
Also, you should keep in mind that when a sentence begins with a number, it is usually spelled out, not written with numerals. So ...
*One hundred kilograms is a heavy weight.
*One thousand dollars is a great sum of money for the boy.
*One thousand dollars is a large amount of money for him.
*One thousand meters is a long distance to walk.