Modi, whose Bharatiya Janata Party (BJP) won the first majority in 30 years earlier this month, was due to be sworn in Monday at a mega-ceremony to be attended by India's neighbours, including Pakistani counterpart Nawaz Sharif."Team Modi" will have 23 full-rank cabinet ministers, down from Singh's 28, while the number of junior ministers known as ministers of state will be reduced to 22 from 43 in Singh's government, the reports said.
NDTV, the CNN-IBN news channel and the Hindustan Times newspaper said Modi would make himself defence minister, while the BJP's most senior woman leader Sushma Swaraj was widely tipped to be foreign minister.
Long-time Modi ally Arun Jaitley was seen as frontrunner for the vital finance portfolio as Indians look to the BJP to live up to promises to boost growth.
Early Monday, Modi confirmed rumours that he planned to slash the size of the cabinet in keeping with his campaign promises of "less government, more governance".
Some government departments would be "transformed into Organic Ministries for quicker pace of work & better coordination between departments," he tweeted.
"This is a step towards Smart Governance and will give us added strength to fulfil the aspirations of the people," he added.
BJP president Rajnath Singh was widely linked to the home ministry job.