Food in your garden so no one starves because there is not enough food for everyone due to rationing.

Its slogan "dig for plenty" was trying to get people to grow food in their gardens.

The picture of a box full of vegetables .With its slogan on the box is trying to get people to grow their own food instead of always relying on ration's to eat.

The size of the slogan is saying.

"GROW BIG not small" the color of the text is dark but the picture is light

which draws your eyes to the text not the picture.