It's Pattern, published 1954, a short story by Fredric Brown.
The aliens (and their spaceships) are gigantic but lack density: if an
alien stomps on a house, it does not destroy it; people inside the
house just see a shadow, as if a cloud covered the sun for a moment.
So, after the initial horrified reactions, humans start ignoring
The story, which is really short, ends with the situation described in
the question: while outside the giant aliens can be seen creating some
sort of clouds, one person asks another (who was working in the garden)
whether she was giving some fertilizer to her plants, to which the
answer is something like "Not fertilizer. Poison." (thus suggesting that the aliens are going to exterminate the humans using those
Many thanks to user ImaginaryEvents for finding the original title. I only knew its Italian translation: in Italy, this story was published in 1961 in Il secondo libro della fantascienza (= the second science-fiction book), a collection of short science-fiction stories, as Questione di scala (= a matter of scale).