I’ve noticed the same thing with my kiddos. It’s like watching a worm crawl. One minute it’s short and chubby, the next minute it’s long and skinny as a string.
with our first child we fretted and considered putting her on a diet - and then BAM! She’s tall and thin.
Luckily she prepared us for the pattern her little brother would show - he was even more extreme and grew extremely fast after the pudge faze.
there’s 4 more that will go through these phases, and I won’t consider it a problem unless they are still pudgy by the 10th grade.