The history of china is a history of expansion. Many non-chinese
from either north or sourth were "converted" into chinese and
the land under chinese control expanded many times in history.
Expanding oversea is a modern concept. Also it was hard to collect
land tax from oversea. But losing peasants because of migration to
oversea definitely will reduce tax collected from mainland.
So it is not a suprise that most emperors discouraged migration
of people oversea. This is different from expanding
