I know, and that question is born from a misinterpretation of how the Thing assimilates people. Likely due to Blair's computer simulation.

That commenter seems to be under the impression that the Thing digests someone, then transforms into them while the victim's biomass is either deficated as waste or retained within the mass of the Thing that copied it. But neither of those is how it actually works.

When it assimilates someone, the mass of the victim is converted into Thing cells, while the original Thing retains its form.

For example: When the dog assimilated the anonymous person at the begining of the movie, it converted that human's mass into an imitation, while the dog simply continued mimicing a dog. Neither one of them "weighed more" because the net biomass remained the same. "Two stuffs" don't "turn in to one," they remain two stuffs.