As I read the list of animals that Abram is to collect for God and the order in which they are listed what stands out to me is that they seem to appear in order in size from largest to smallest. The significance of this size order is obviously... well, not obvious!
In the previous verse Abram had asked the Lord GOD "Whereby shall I know that I shall inherit it?" That, I believe establishes the prevailing context and I suspect that the trend of the animals downward in size is intended to help answer that question. But there is much, much, much more to the story.
The animals were all animals that would be, under the Torah, ceremonially clean animals, suitable for sacrifice. Abram was to provide the animals, divide them, lay each piece alongside the other and, apparently, protect the animals from birds.
Abram was to divide the three large 3 year old mammals in half but the birds were not divided. The Jewish Encyclopedia shows that the apocalyptic writers think that they refer to the "four kingdoms" of Daniel:
His [IE: Abram's] prophetic vision (Gen. xv.) furnished especially grateful material to apocalyptic writers, who beheld foreshadowed in the four different animals used for the covenant sacrifice the "four kingdoms" of the Book of Daniel (see also the Midrashim and Targums and Pirḳe R. El. xxviii; compare Apocalypse of Abraham, ix.).
I think the Daniel references in apocalyptic writings is significant but a little beyond my ken at the moment.
While I seem to be oddly alone in this view, it looks to me that we have some intertextuality in "To the Hebrews":
YLT Heb 9:
17 for a covenant over dead victims is stedfast, since it is no force at all when the covenant-victim liveth 18 for where a covenant is, the death of the covenant-victim to come in is necessary,
The covenant is "stedfast" because the ritual "seals the deal" and henceforth is cannot be changed except on pain of death of the one who provided the animals (normally the beneficiary):
Gal 3:15 KJV - 15 Brethren, I speak after the manner of men; Though it be but a man's covenant, yet if it be confirmed, no man disannulleth, or addeth thereto.
So for Abram's part, he was to provide the various animals and chase away the birds. These, I believe alludes to the various sacrifices that Abraham was to offer in order to be cleared of any transgression of the covenant. This was to indicate that it was the life of himself and his descendants that was on the line if they break the covenant but the sacrificial system would allow the covenant to remain in tact when maintained by frequent expressions of remorse through animal sacrifices.
So Abram had from God a covenant that Abram and his offspring were responsible to maintain. The chasing away of the birds symbolized his required "works". If he and they abide in the covenant they will inherit the land. This was Abram's works oriented assurance of salvation.
Seven chapters later, however, YHVH appears to him again and asks him for the ultimate sacrifice: his monogenhs son Isaac. Abraham, knowing that all his hopes and dreams rested on the vision he had been given of a works covenant. And yet, seemingly, God was asking him to cut off his line making it impossible for his seed to inherit the land. But he trusted YHVH so implicitly that he presumed that he believed that God would fulfill his promises if he kept his part, even if it meant raising his son from the dead!:
Heb 11:17 KJV - 17 By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten son,
And what did Abram say to Isaac?:
Gen 22:8 KJV - 8 And Abraham said, My son, God will provide himself a lamb for a burnt offering: so they went both of them together.
Heb 11:17 KJV - 17 By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten son,
Rom 4:13, 16 KJV - 13 For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith. ... 16 Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,
And of course he did provide the Messiah, the "Lamb of God". Now, because God provided the Lamb it was God's responsibility to cause his descendants to inherit the land.
And what did YHVH say to Abraham?:
Gen 22:16-18 CSB - 16 and said, "By myself I have sworn," this is the LORD's declaration: "Because you have done this thing and have not withheld your only son, 17 "I will indeed bless you and make your offspring as numerous as the stars of the sky and the sand on the seashore. Your offspring will possess the city gates of their enemies. 18 "And all the nations of the earth will be blessed by your offspring because you have obeyed my command."
God added more assurances and said the whole world would be blessed through his "seed". The primary assurance was his oath.
Deu 31:21 ASV - 21 And it shall come to pass, when many evils and troubles are come upon them, that this song shall testify before them as a witness; for it shall not be forgotten out of the mouths of their seed: for I know their imagination which they frame this day, before I have brought them into the land which I sware.
Jer 11:5 ASV - 5 that I may establish the oath which I sware unto your fathers, to give them a land flowing with milk and honey, as at this day. Then answered I, and said, Amen, O Jehovah.
Eze 47:14 ASV - 14 And ye shall inherit it, one as well as another; for I sware to give it unto your fathers: and this land shall fall unto you for inheritance.
Perhaps the diminishing size of the sacrificial animals indicated that the system of animal sacrifices was going to fade away.
