(NOTE: it is not about two poems, it is about a poem and a painting)
In the first place, Williams' poem begins with a direct reference to Brueghel's painting ( According to Brueghel ..). The poem is actually a description of the painting and in the end, there is an attempted explanation of its meaning.
In the beginning, Williams descibes the image (it was spring, a farmer was ploughing, near the edge of the sea e.t.c.). If we observe the painting, we can easily see the farmer, who is in the foreground and is wearing a bright red garment. We can see that he is ploughing his field near the edge of the sea and that it is actually spring.
Williams mentions Icarus from the very beginning (When Icarus fell...) whereas it is not as easy to discern Icarus when we look at the painting for the first time. Brueghel has not chosen to reveal the meaning of the painting so easily. Icarus is not the main figure in the painting, as he is placed in the right corner and we can only see his feet because he is already falling in the sea. On the other hand, the image of the farmer attracts our attention (he is placed in the centre and the painter intentionally uses a very strong colour - red - to guide us there). There are also other figures in the painting, who do not appear in the poem: a fisherman who is fishing from a cliff right above the falling Icarus, a shepherd with his sheep looking up in the sky and a ship very close to Icarus, where we can actually see some sailors - one of them is climbing a rope ladder.
All the above are not described in the poem, but they are contained in the phrase ...the whole ......concerned with itself , which denotes the indifference of everyone to this dramatic event. The fact that the poet uses the words 'unsignificantly' and 'unnoticed' also is similar to the way the painter placed Icarus - he is unsignificant, so he is placed in a small corner of the painting.