Lighter would make it expand slower.
Think of an ice skater with his/her arms out.
When they pull their arms in, they spin faster.
Now, take an ice skater holding watermellons. The ice skater spinning with the watermellons spining at the same rate as the one without, would spin faster when they pulled the mellons in, because the extra weight allows the skater to have more potential energy, which turns to kinetic energy when the arms are pulled in.