Tirian is surprised to discover a whole world inside the tiny stable.  What does he find growing on the trees?  

A grove of trees was thick leaves and there peeped out the gold or faint yellow or purple or glowing red of fruits. And they also ate the fruits which no one could describe the taste.

What does Lucy say “once had something inside it that was bigger than our whole world”?  

Lucy wanted to express that the world in a stable was bigger than their own world. Looking from the outside, it just was a stable (maybe a frightening room).

But when they were in the stable, there was the blue sky overhead and grassy country spreading as far as they could see in every direction. The stable was bigger than their imagination. and the stable seen from within and the stable seen from outside were total two different places. 

Lucy was surprised by the stable’s miracle and she enjoyed herself in the stable.

Why can’t Aslan do more for the Dwarfs?  

Aslan had given Dwarfs food and drinking. But they still said they didn’t let anyone take them in. The Dwarfs had chosen cunning instead of belief. Their prison was only in their own minds, if they could take out by themselves, no one could help them or do more for them.