I noticed that my 6-month-old has become quite lighter whenever I carry him even though he didn't lose weight. He's not that as siksik anymore unlike before. How come? Is that normal?

I think that's part of a child's growth process. As long as your child isn't in pain or exhibiting visible discomfort (which may be signs that something is wrong), then you and your baby should be fine. But if you suspect that there really is something that isn't right, trust your maternal instincts and consult with a doctor.