Light Envelope makes no sense

How much do explosives weigh?

OK, I am not going to research this particular topic, so I might be wrong. Imagine that!

I would still at least assume or imagine that explosives have SOME kind of weight. They talked about 'megaton' bombs back in the day, so the weight must have some kind of correlation as to how powerful a bomb or explosive is.

This makes sense.

We always hear, even in movies, someone saying '..enough C4 to blow up..'

This means the amount of C4 must be crucial in creating big enough an explosion, right?

Now, look at the scene near the beginning, where the antagonist woman goes and gives the envelope to the secretary or whatever. It's SO light, she can not only hold it in one hand, but judging by how she hands it to the secretary, and how the secretary receives it and handles it, it CAN NOT weigh more than 'a few pages of paper'.

I am asking the reader here; how realistic is it that an explosive that weighs only the same as a 'few pages of paper', can decimate half the floor of a big building, and then have enough 'leftover typically hollyweird gas cloud explosion' to spew over from the windows the way we're shown, when the antagonist woman says something like 'someone better call the police'?

How realistic is that an explosive _THAT_LIGHTWEIGHT_ can create an explosion _THAT_MASSIVE_?!

I would bet 97 euros and 82 cents that the envelope being that lightweight makes no sense.

(What? I am not made of money..)