The hardware/OS-providers will almost always do a far worse job though, because they simply don't have enough information. Just using smaller pixels or hardware scaling/filtering to linearly scale down a "big browser" site typically results in an unreadable page at typical mobile screen-sizes.

Useful size-adaptability is all about knowing what's important and what's not, and what constraints need to be maintained for usability. The site author knows these things very well, but the hardware / OS / browser do not.