Because the data is compressed to reduce download time, and inflated at the host computer.

Imagine that you live somewhere that for every sentence, they they make you append "I love our great and mighty country and our wonderful leader."

If you had to dictate your work over the telephone, you'd probably abbreviate the repetitive part to "I love the country, etcetra."

Then when the secretary typed it, she would expand the crazy stuff herself.

Everything except purely random data can be compressed, almost always at least 50%. Windows expanded from 11G to 65G, so it was compressed to 1/6 he size.

Text can usually be compressed to 1/15 the size. One reason is that all the spaces can be replaced with a very short symbol (string of bits). Rare letters like Q and Z are replaced with a long string of bits.