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?

Maybe that's because your baby has become more active than before, Babies tend to burn lots of calories as they reached 6 months because they start recognizing more of their physical capabilities. Some babies even lose weight at the onset of 6 months. Best thing to do is try consulting your pedia