Tell you what, let’s apply some very basic math, physics and engineering to this.
First of all the thing to think about when considering loads is not the volume of stuff but the mass. Next you have to consider how the load on one floor changes when the floors above it fall down onto it, as opposed to just resting there in their normal static state.
As I said before, there’s no engineering reason that the lower floors HAVE to be stronger than the upper floors (as long as each floor can hold up AT LEAST what’s above it that’s the bare minimum, you could certainly build all floors the same or even make the upper floors stronger than the lower ones) but let’s take it as a given that that’s how they built it for economic reasons (with a primarily steel-framed building it may make sense, whereas with reinforced concrete a lot of the cost is in the forms so you try and keep them uniform).
Let’s say that the 90th floor is where the plane hits.
Now we throw a catastrophic failure into the works and the 90th floor loses enough strength (members destroyed by the plane crash, others bent out of shape which lessens their capacity, load redistribution from failed sections of the structure which overloads adjacent supports, weakening of the steel due to the heat) so that it fails and those 20 floors drop down (we’ve just kicked the legs out from under the stack).
Think about what happens if you drop a weight some distance and try to stop it quickly. You could for example probably pick up a 100 pound kid, right? If that kid jumped off a 10 foot tall garage roof could you catch him and prevent him from hitting the ground? Much harder and as you appreciate basic high-school physics you can do the math just as well as me to see why it takes a lot more force to stop a fast moving object in a short distance than it takes to hold it up against 1G.
Roughly speaking if you let something accelerate at 1G over some distance D and then you try and stop it in D/10, you’re going to need to apply an average deceleration of about 10G. Simple physics. The shorter the distance you try and stop it in, the larger the deceleration you need to apply. Grab a pencil and work on d=1/2at^2 and d=v(ave)*t if you want to check me.
So going back to the building, 20 floors drops about 10 feet or so and smacks into the 89th floor. The 89th floor was designed to hold the 21 floors above it, plus a safety factor. Safety factors aren’t as simple as people think (my minor focus was probabilistic methods in civil engineering and I’ve derived some of the LRFD numbers used in the building codes) but let’s just say that the 89th floor was strong enough to hold up twice the weight resting on it - so that’s 42 floors worth of stuff.
How quickly does that chunk of building slow down? I used 1 foot, just to pick something in the right ballpark - it certainly can’t be much more because that’s thicker than the floor/ceiling slabs anyhow; if it takes longer than that to stop the roof just caved in. If that mass of stuff stops in 1 foot it applied a force of around 10 times its own weight to the 89th floor - so that’s about 210 floors worth of weight as an impact load, in a fraction of a second.
Well that’s a problem because 89 was never designed to hold anywhere near that kind of load. So what happens to 89? It gets crushed and now we’ve got 22 floors worth of stuff dropping onto the 88th floor. This will keep happening all the way down.
The fact that it was only the top 10% or 20% of the structure that started the collapse isn’t the limiting factor, and neither is the fact that the 1st floor was quite capable of holding the 109 floors above it. It’s the impact of a floor suddenly dropping onto the floor below that overloads what each floor was designed to hold.
So you look at the lowest floor and say “How could 10% of the building cause collapse in the level that can hold up 100% of the building?” It’s because nearly 100% of the building falls onto that level in a manner that multiplies its effective weight by a large factor which outstrips whatever strength the lowest level had.
No fancy concepts here, no hand-waving, no obscure engineering formulas.
Now things could have turned out differently if that falling mass had sloughed off to the sides instead of accumulating as it went down but that’s obviously not what happened - despite all the debris and dust you see flying around as the towers collapsed the bulk of it went pretty much straight down and that’s your collapse mechanism.
ROFLMAO 
