Isaiah 9:3 is organized in a fairly neat chiasm, with a triple use of the word “joy” ( samach ) in the center:

A. You multiply the nation

B. You make great their joy ( simchah )

C. They rejoice ( samach ) before you

B’. As the joy ( simchah ) in harvest

A’. As men are glad ( giyl ) in dividing the spoil

There is another structure working here as well. The first two lines begin with verbs in the second person singular; grammatically, the A-B could be seen as an A-A’. The last two clauses both begin with the comparative particle k- , and so B’-A’ could be seen grammatically as B-B’. It is also significant that the “joy” words work in a 3 + 1 way. The same root is used three times, but in the final line, a synonym is used. They rejoice three, yea four times.

A few other observations. First, the first lines link the increase of the nation, the expansion of the population, with the increase of joy. Second, the “nation” here is goy ; that makes contextual sense, since 9:1 speaks of “Galilee of the goyim ” but it also works to blur the distinction between Israel and the nations. Third, chiastically, the increase of the nation is linked to the division of spoil. The nation is perhaps being multiplied by “spoiling” people from the Gentiles. Finally, the center of the verse is joy before Yahweh, a liturgical setting.