Experts say the move is likely part of long-mooted rationalization plans, which have included changing the PLA command structure so it less resembles a Soviet-era model and spending more money on the navy and air force as Beijing asserts its territorial claims in the disputed South and East China Seas.
Soon after Xi's announcement, the official Xinhua news agency published a long article quoted soldiers as supporting the decision.
Each branch of the armed forces believed the cuts would raise quality standards, Xinhua said.
Commentaries in the PLA Daily newspaper have since warned that the reductions would be hard to carry out. Chinese state media often run commentaries that reflect the official line of the institution publishing the newspaper.
The cuts come at a time of heightened economic uncertainty in China as growth slows, its stock markets tumble and the leadership grapples with painful but needed economic reforms.
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China has previously faced protests from demobilized soldiers, who have complained about a lack of support finding new jobs or help with financial problems.
A protest by thousands of former soldiers over pensions was reported in June, although the Defence Ministry denied any knowledge of the incident.
The PLA is already reeling from Xi's crackdown on deep-seated corruption in China, which has seen dozens of officers investigated, including two former vice chairmen of the Central Military Commission.
Barely a week after the Beijing parade, the PLA newspaper said the troop cuts and other military reforms Xi wished to undertake would require "an assault on fortified positions" to change mindsets and root out vested interests, and that the difficulties expected would be "unprecedented".
If these reforms failed, measures still to come would be "nothing more than an empty sheet of paper", it said.
It did not give details on the planned reforms.
But state media has said they will likely involve better integration of all PLA branches. As part of this move, China's seven military regions, which have separate command structures that tend to focus on ground-based operations, are expected to be reduced.
There had been no previous suggestion big troop cuts were planned.
Troop entertainers to go
Another commentary in the PLA Daily published a week later detailed the kind of opposition Xi faced.
"Some units suffer from inertia and think everything's already great. Some are scared of hardships, blame everyone and everything but themselves ... They shirk work and find ways of avoiding difficulty," the commentary said.
A second government source, who is close to the PLA, said military song and dance assemblies, which traditionally entertain troops, would be the first to go.
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"The defense budget will not be cut. It will continue to gradually increase," the source added.
China's military budget for this year rose 10.1 percent to 886.9 billion yuan ($139.39 billion), the second largest in the world after the United States.
Some retired Chinese generals have supported the troop cuts.
"A bloated military can only cause ineffectual expenditure and forfeited battles," retired Major-General Luo Yuan, a prominent Chinese military figure, wrote in the Global Times newspaper three days after Xi's announcement.
Xu Guangyu, a retired major general and now a senior army arms control advisor said: "Our country's military needs to take the path of modernization ... These force reductions are an effort to stay on this path and increase quality not numbers."