What is so difficult about this assumption? If person A's income is just twice as big as person B's but every thing A purchases costs 4 times the amount of B's purchase of the same goods then of course A is worse off than B. Got it?
Here is how much of their income Indians spend on food.
As per this link
http://www.statistics.gov.lk/HIES/HIES201213BuletinEng.pdf Sri Lankans spend 38% of their income on food.
So that should tell you who is left with more money for discretionary spending. Also that should tell you if Indians are malnourished, a much abused word given even the obese are categorized as malnourished, it is by choice.
I would take those figures about poverty in India with a huge pinch of salt given the size of our black economy and the massive under reporting of incomes across all spectrum of the society.
As to how does a slum dweller also in India can live like a middle class in Sri Lanka, it is simple even a slum dweller earns upwards of 20 K in India. Food and other expenses are much cheaper and given that slums are an urban phenomenon, the slum dweller neither lack access to hospitals or educational institutes in India.
Actually, you should re-look at the numbers once again. While the cost of living is much lower in India compared to USA, USA has an income that is 487.49% higher than India which makes its purchasing power 60.73% higher than India whereas Sri Lanka has no such luck. So my argument still holds and your reading ability of given data sucks.