As other mentioned big airs are a combination of speed, lots of power in your kite, board pop, kite ''sending'' and perfect timing.
Now from observing newbies learning to boost the board pop has to be the weakest link of all. Being overpowered, sending the kite and going fast on your board is something everyone can do. But if you can't hold your edge correctly to load and pop then you will get low height pendulum style jumps.
When learning to jump it's best to trade in some speed for some pop, i.e. ride slower but
edge harder while sending the kite (this is when the timing is important); you'll be surprised how much higher you can go even with half the speed (assuming you have lots of power in your kite of course).
It pays off as others said to spend time working on your load and pop
without sending the kite and keeping it at 45' degrees. You should be able to pop at least 1meter off the water without sending the kite at all, that's a good indication of how good your pop is. Pro's probably pop 2 meters off the water without sending the kite, perhaps even more.
From observing pros jump it's mostly who can hold their edge the strongest at the highest speed that goes higher. That's much easier to do in flat water or when using a wave as a kicker. It gets much more difficult in choppy water.
Here's short edit of a boost in flat water, slowed down at the load and pop. Could do it better myself and go even higher, but you get the picture.