“impossibly” is just the name of the tag. It is not meant literally; it does not mean living humans can’t exist like that. This has been discussed in the past and the name decided not to be changed.
Informally, each breast:
“big”: more than a handful but smaller than a head
“huge”: bigger than a head, smaller than a ribcage
“impossibly”: larger than a ribcage
…with interpretive allowances made due to varying art styles having different proportions (eg. a cartoon style might draw heads larger than ribcages, or hands bigger than heads, which throws a strict interpretation out the window)