There was someone on youtube a while back that made a CG version of this scene with the explosion moving away and it doesn’t work. The thing with an explosion that moves away is that it gives the impression that its getting smaller and it lessens the visual impact. Thankfully Star Wars really doesn’t stick to real world physics when it comes to FX and goes for a more visual impact than real world.
Ahh you’re absolutely right - the visual impact would be greatly lessened were the explosion to get smaller.