The first two aliyot of Parshat Ki Tisa are pretty long. Then the rest are all very short. I was told that the reason for this is that we want to give the Levi the aliyah that praises him, when everyone else sinned with Cheit Ha'Egel (the Golden Calf). OK, that makes sense. But why does it start from Ki Tisa, and not later (allowing it to be spread more evenly)? What's so significant about starting with Ki Tisa?
I have not been able to locate anything specific to addressing the choice of the location of where Ki Tissah "should" start. However, I recommend reading pp. 81-90 of this paper.
The premise of the paper is on when and why parshiot are doubled or even "halved". The pages I mentioned discuss the general parsha length and offer several opinions from Sa'adia Ga'on and Ramba"m on how to "manage" parshiot. The overall assumption, though, seems to be theat the designation of the parshiot is Mesorah.
The paper is very long, and only a small section may apply to your question. Nonetheless, I find the whole paper fascinating, and I hope you will, too. It's comprehensive and well written, even if it may seem confusing at times.