As far as weights go, the 707 and the 767-200s that hit the towers were fairly close in size, weight and fuel capacity and that is demonstrated by both towers surviving their impacts. The problem was the fire and specifically the inability to fight the scale of the fire effectively.

And insufficient modelling for the heat of jet fuel mixed with office supplies which severely weakened the structure allowing it to pancake in a cascading fashion from the first support columns to buckle.

The designers also assumed that the airplane would be moving pretty slowly if it was so close to the ground, either departing from or landing at a nearby airport, not coming in at near top speed.

First of all, the WTC towers did withstand the impacts but collapsed due to the ensuing fires which weakened the remaining core columns (notably not due to collapsing floors, which did not in fact occur, according to Thornton-Tomasetti who have modeled the impacts and ensuing collapses as part of their forensic investigation commissioned for the insurance claim litigation).

Leslie Robertson, who lead the WTC structural engineering team along with John Skilling, is on tape making the 707 claim, but no surviving documentation has been uncovered to reveal the specific scenario that was modeled or the calculations used.

An important reason why the towers survived the impacts at all was the presence of the two hat trusses, which allowed the increasingly unsupported loads to be redistributed to the surviving exterior columns. Najib N. Abboud of Thornton-Thomasetti has given a detailed account of the findings of the forensic investigation that's available on Youtube[1].

[1] https://youtu.be/b2wimsBPmYI?feature=shared