MoparX's explanation couldn't be better, seems to me.
Note that there's a minimum level for coolant in the overflow tank - even when rad is full - so the system won't suck air.
(Look at any OEM tank, you'll see "minimum" & "maximum" fluid levels).

Opposite of expand is condense, though in some cases that's not the most articulate, where shrink may better apply...

decrease in size, number, or range.