We've sailed on the Magic 10 times...sometimes full, sometimes close, sometimes not at all. The last cruise, a whole area of cabins was closed off for refurbishing...so although the cruise was listed as "full," it was much less than it's nearly 2700 guest capacity.
"Full" is a strange word. There are 800+ cabins available normally. All cabins technically hold at least 3 people. Since we are a 2 person family, selling us a cabin automatically reduces the ship's capacity by 1. Ok, so one person isn't a big deal, but if you repeat this several hundred times, a "full" ship may have 2400 guests on board. Also, when you make a reservation, a spot is automatically reserved for any children in the "age appropriate" kid area. Thus, there are times when I could make a reservation for 2 adults, but not for myself and my child (as her age group is full).
All that said, we've only had one experience when the ship really seemed crowded. That was the maiden voyage, when
DCL really hadn't figured out the crowd control issues yet. There are now so many activities, places, etc. for people to be that we can always find a quiet spot when desired. We do like to travel "off season" as the kid older kid population is much less at those times.