To use a pun, these are ballpark figures to give people a general idea, not an exact measure. Also about as heavy as a midsized car. Just gives people a general idea.
Using “ballpark figure” to explain “football field” is my favorite thing today.
I never quibble with these types of comparisons, except for Libraries of Congress when trying to convey the amount of data something can hold.
I have no idea how large the LoC is. My touchstone for data capacity is MP3s (128k/48kHz, of course :)
As far as football fields goes, I’d think it works best because gridirons and pitches are roughly the same size. Almost everyone has had experience with at least one of those, right?
Akin to stadia.