The Chinese Treasure fleet, commanded by Zheng He, was sent to carry the message of the grandeur of China, and offer tributary status to those states it visited. The Chinese did not seek trade or booty; much less to impose their values on other cultures; in fact they normally presented gifts, etc. of far greater value than they received in return. Interestingly a monument was erected on the island of Ceylon (present day Sri Lanka) to three separate deities, including Buddha and Allah, in three separate languages. The Chinese were determined to be politically correct.
The Portuguese came to trade, Christianize and "civilize" by whatever means possible. Vasco da Gama was at first rebuffed when he attempted trade in India; so in order to get the attention of the local ruler, he captured several local fishermen, dismembered them, then sent their heads, hands and feet in a basket to the ruler with an admonition that more was to follow if he did not relent. Trade terms imposed were unequal to say the least. Additionally, Jesuit Priests came with the Portuguese to convert the local people to Christianity.
As a final comparison, China shut itself off from the world in the Great Withdrawal, and the reports of Zheng He's voyages destroyed. China considered itself the Middle Kingdom and intended to remain that way. The Portuguese never left until expelled by other European powers. Two excellent sources are Daniel Boorstin, The Discoverers, and Giles Milton, Nathaniel's Nutmeg.
The Treasure Fleets were commanded by a powerful eunuch admiral named Cheng Ho. Cheng Ho was born around 1371 in China's southwestern Yunan Province (just north of Laos) with the name Ma Ho. Ma Ho's father was a Muslim hajji (who had made a pilgrimage to Mecca) and the family name of Ma was used by Muslims in representation of the word Mohammed.
When Ma Ho was ten years old (around 1381), he was captured along with other children when the Chinese army invaded Yunan to take control over the region. At the age of 13 he was castrated, as were other young prisoners, and he was placed as a servant in the household of the Chinese Emperor's fourth son (out of twenty-six total sons), Prince Zhu Di.
Ma Ho proved himself to be an exceptional servant to Prince Zhu Di. He became skilled in the arts of war and diplomacy and served as an officer of the prince. Zhu Di renamed Ma Ho as Cheng Ho because the eunuch's horse was killed in battle outside of a place called Zhenglunba. (Cheng Ho is also Zheng He in the newer Pinyin transliteration of Chinese but he's still most commonly called Cheng Ho). Cheng Ho was also known as San Bao which means "three jewels."
Cheng Ho, who was said to have been seven feet tall, was given greater power when Zhu Di became emperor in 1402. One year later, Zhu Di appointed Cheng Ho admiral and ordered him to oversee the construction of a Treasure Fleet to explore the seas surrounding China. Admiral Cheng Ho was the first eunuch appointed to such a high military position in China.