As others have said, China doesn't have any need economically to expand as a colonial Empire. Europeans expanded colonially due to Islamic powers blocking their trade /with/ China - they wanted China's resources (and later, once they discovered the resources of the Caribbean and such, that provided a further impetus).
But it's telling that even in the 19th century, when the European powers were arguably at their height, they were trading at a deficit with China, as the only good they could provide that China wanted (barring a few individuals liking mechanical clocks as a novelty) was Silver from the Americas.
If you lived in an area that produced all the luxury goods you needed, and at enough of a surplus that you could make an absolute fortune trading the excess away, why would you feel the need to expand colonially?