This is often said (also about the Viking age) and it seems such an incomplete explanation to me. Normally if more people are born than the land can feed, then vulnerable people (eg babies and kids and elderly) die at higher rates until the balance is restored. Why would Scandinavia be different?

They didn’t just have the motivation to leave, they also had the opportunity.

Their longboats made them amphibious. Their ships were durable enough for the open ocean, but still able to land on any beach, or sail fair upriver.

There are often cultural elements in that balance. Many societies had cultural adaptations that lowered fertility under normal circumstances but enabled faster repopulation after a crisis. But if there was an outlet for excess people, the base reproduction rate could be higher.

For example, the Roman Republic had adapted to constant warfare and the steady stream of casualties caused by it. And then the Late Republic got into trouble, because Rome had become too good at war, and the required casualties didn't happen.

The Irish solution was mass emigration and blaming the English.

They had too many births before the potato famine -- it's the reason why it was such a disaster in Ireland compared to the rest of Europe -- and they continued up until a few decades ago. This happened during a time when it was unusually easy to move to "empty" land and colonize it.