Well I’m no expert but this is t’internet so semi informed opinion is as good as 😉
Thunder is caused by the explosive expansion of the air around the lightning stroke as its superheated to around 30,000 degrees C. My guess is that fork lightning that strikes the ground is passing through and heating much denser air (bigger bang) than it would be at altitude.
Thunderclouds (Cumulonumbus) can reach the top of the troposphere (12km up I think) and lighting up there would definitely make less of a bang than in the thinner air.
On top of this you also have to factor in that a lightning stroke 10km up in the atmosphere is immediately 10km further from your ears than one the strikes the ground nearby