70 years back India and China were very much similar in terms of GDP and population. After Mao failed economic policies like land collectivization, China opened its economy in 1970’s and moved towards capitalism. At that time China was the only Communist country which applied capitalism.
Whereas India on the other hand opened its economy in 1991, even after being a Democractic country India chose socialism.
In 1979 China re-established diplomatic ties with USA under the leadership of Deng Xiaoping which brought lot of foreign investments from US due to China’s cheap labour and low rental cost.
From the beginning of 1980’s China saw double digit growth rate for the following 15 years and it was the time when it over took India.
Population: Although India and China have same population; China utilized its population wisely. Chinese govt promoted skill development among youth and encouraged its population to join manufacturing sector.
As the large number of people started choosing manufacturing sector, slowly during 1990 and early 2000’s China became World’s production house by simply imitating top brands and developed countries took advantage of this and established their factories in China.
Technology transfer: As lot of developed countries started opening companies in China the then Chinese govt started asking for technology transfer so as to open production house and many companies accepted. That’s why today China had its own Google, YouTube and Whatsapp. Its like “Give a man a fish, he will eat for a day. Teach a man to fish he will be fed forever.”
Long term vision: Democractic countries release manifesto for every five years; whereas Chinese govt release 15 years Manifesto and they follow it religiously. Also one more thing is noone in China questions govt decisions like in India where even small land acquisition by govt becomes a national news.
Economy manipulation: China in early 2000 took 3 times the loan of their then GDP from Chinese central bank without any assurance of paying it back again. Its called GDP to debt ratio which is now 300%, China invested this money to build world class top infrastructure for foreign companies, developed many cities and created Economic zones.
There are also other factors like China saw a century of humiliation which paved the path and hunger to develop and compete with western world whereas we Indians are forgetting what British did to us.