if India and other nations don't like how the UN is structured, unfair/injust, then why not leave and make their own? Is somebody forcing them to be in the UN or to partake in the current economic system? If they are being forced and they acquiesce to the demands then they are probably not powerful enough to earn a seat.
Having many people is not reason to have a seat, nor is having nuclear weapons....the UK and France don't have many people. Israel, Pakistan and North Korea all have nukes, many other nations could build nukes on short notice.
You don't change the laws just because more people arrive.