If China had any legitimate competitors in their area, or lacked resources or space then expansion would likely have become a priority.
As has been said they did expand a great deal until they had no need for more room, resources etc.
Spreading across the world can be more trouble than it is worth (ask Spain) and the logistics of China getting to, say, North America make such a venture extremely expensive, but also far less likely to succeed.
One of the reasons Western Europeans colonised in the way they did was because there was nowhere else to expand to at the time, and they had excess population.
Give China less territory to expand into, more dangerous neighbours and naval technology which allows quick and safe oceangoing travel then it could happen. As it was China had a wonderful position, territory and was the local superpower, not much cause to expand from that is there?