Projects get completed right before their deadline (if you're lucky). Design complexity increases with more time (even for simple bugs). The principle applies to more resources than time – programs often are written (subconsciously) to make use of as much CPU and storage are available (see Google Chrome or Slack).
I'd write fluff in grade school to reach an arbitrary 500-word requirement. Now – I try to keep it as short as possible while expressing my point.
Work expands to fill the space.
Constraints can sometimes inspire creativity. Sometimes I'm most productive in the short blocks of time between tasks on my busiest days.