Getting Too Big

My oldest has been sleeping on the couch this summer because it is too hot on the top bunk (we have no air conditioning). He is an amazing sleeper. He sleeps through the hubby getting ready for work, me leaving on my runs and coming back, his brother and sister waking up and eating breakfast, and the dogs barking most of the time. This is the same kid that woke up every two hours at night until he was two or three and didn't really sleep through the night until he was five. The same kid that, as a baby, was really hard to get to sleep in the first past and way too easy to wake up, too.
The other day I went out to the living room and watched my big 11 year old sleeping, and I realized he is getting really big. Yes, he is still a bit small for his age, but compared to a year ago, he is big. What happened to my kid that grew slowly? Am I really ready for this?