While they watched, the cloud grew visibly larger, or perhaps closer.

The cloud grew to the size of his head, then dissipated.

The clouds grew grayer and it began to snow again.

The clouds grew like quick crystal, around threads of fire.

As I understand what you've said, the cloud is growing in its apparent size because it's getting nearer to us.

The clouds thickened and grew black as the day wore on.

The black cloud was growing in size; already it blotted out half of the chamber.

The clouds above seemed to lower and grow dark.

The cloud was growing to the size of a fingernail now.

The cloud had grown from a pinpoint to the size of a large coin.