Cool link.
You need to first learn how to use a pocket calculator.
In the thread you reference, Matt puts the mass of WTC7 at 267,000 tons (imperial). 17% of 267,000 tons is 45,390 tons, not your value.
Secondly, you need to read the entire thread, including replies:
Matt almost certainly overestimated the mass ot the twin towers by a considerable factor. Since Matt bases his estimate for WTC7 on the estimate for the twin towers, he gets the WTC7 mass too high by the same margin.
Also, he factors in the mass estimate for WTC3, which has added mass because of the different structural layout, he shouldn't have done that at gone with 252,000 tons to start with, where you chose to go with the 281,422 tons, arrived at by extapolating assumptions for the Marriott Hotel.
I have my own estimate here:
http://oystein-issues.blogspot.com/2010/09/gpe-of-building-wtc7.html
I estimate 116,000 metric tons, which is approx. 130,000 imperial tons. Only about half of what Matt estimated. 17% of that is 22,100 tons
Thirdly, we are not talking about 17% of the entire builing, only 17% of a part of the outer shell, which in turn is only a part of the entire building. If we assume that the outer shell is approx 1/3 of the total building (core and floors being the other thirds, as a first rough approximation), and that only the upper half of the north face was visible, we are talking about only 1/6 of 17%, or 3% of the entire building, or about 3,500 tons.
So yes, 3,500 tons is a mere pittance, compared to the 116,000 tons that fell.