Telling kids "you're getting so tall!" is just as rude as saying "you've gained so much weight"
Commenting on other people's bodies is rude, full stop. The fact that this is a normalized phrase to say to kids is not okay. You are putting the child in SUCH an awkward position - what are they supposed to say back to that??
If you haven't seen a child into awhile, you can ask them about school, sports, games, friends etc. Don't open a convo with them about how their body is changing 🤢 Obviously they can't help it, and it serves no one to comment on it