Why do some kids look very skinny in puberty and BAM they get thicker?

I was always a big, heavy kid. I'm still big. My doctor got on my butt about my weight gain. Now I'm 302lbs and I'm 5'10.
My guy friends were scrawny and now their thicker, well not thick but just more broader. Which I'm not going to lie, sexier. My best friend was skinny and now she is slim but curvy and she loves the way she looks now.
She said, "I hated being scrawny! Thank you puberty for blossoming me into a WOMAN! Puberty is a blessing."