So, the first time it was built (by Ibrahim or Abraham) it was much more rectangular. You see the small white wall at its side? That part was included in the Kaabah. Also it was about half as tall as it is now and the door was lower to the ground (like real doors) .
Then after many centuries, there was a flood (since Makkah is a valley). In the same time frame, there was a fire. Both of these and its age caused the structure of the Kaabah to be worn out. So, the people of Makkah, the Quraish tribe, decided to rebuild the foundations.
At this time, coincidentally, there was a shipwreck of a vessel at the nearby port city of Jeddah carrying a lot of building materials and engineers to build a church in Yemen. So, all the materials was being sold to the highest bidder and the engineers were willing to work (to get enough money to go back presumably).
The Quraish pooled their money and bought all the things they could collectively afford. They realized that there wasn’t enough material to cover all of the old foundation. So, they decided to make the design shorter and build a small wall to mark the rest of it in hopes that in the future they could build the rest of it when they got enough money and materials. They also built the door higher up perhaps to prevent floodwater from reaching it or to restrict access to the inside.
That was my instant take, yes. It wasn’t there before. But now they got a needle’s eye for all the camels to pass through.
Or it’s an information kiosk! “Ask us anything about the Cube! We also guess your weight for a fee!”
I checked and apparently it was there before, there’s actually an interesting story behind it:
https://www.quora.com/Why-are-all-sides-of-the-Kaaba-different-sizes-Is-there-any-reason-given-in-the-Hadith-It-seems-to-an-outsider-anyway-please-educate-me-to-be-haphazardly-built-with-no-real-plan
That is fascinating, and thank you! I will read the sources, and then come back here and berate you if they are not up to code! <3