WTC collapsed because the towers were built "on the cheap" the towers should have been built to a shorter height, around 70 stories
Why did a third building; 7 World Trade Center, a 47th story building implode?
Because, at 47 stories, they didnt build it tall enough, according to your silly theory.
Seriously, that building burned for six hours after being riddled with flying beams weighing hundreds of tons and assorted other burning debris. These set entire floors on fire.
In addition to the obvious problems going on that prevented the usual fire departments response, there were also 5 tanks of diesel fuel in the building holding as much as 42,000 gallons of fuel for backup generators, such as the mayors command center.
BTW, the steel beams in WTC7 (which wasnt built until 1987) were treated with fireproofing material, and the rooms holding the diesel tanks were further fire-proofed.
"Any structure anywhere in the world, if you put it in these conditions, it will not stand. The buildings are not designed to be a torch."
- Silvian Marcus, structural engineer
Unfortunately the collapse was shown on TV from the north side, where the fire wasn't, and as a result a lot of people think there was something mysterious/suspicious about #7 falling down. Here's the front of the building, seen from the river:
Now that's a bad fire and, from the angle of the sun, it was still early in the day.