There was a bigger muslim population in India however with the population transfer after partition of India muslims moved to Pakistan and Sikhs and Hindus moved to India. Bangladesh which historically a part of India was called East Bengal and/or East Pakistan during the exchange. It became independent in 1971.

Haven't seen much here yet, so I'll try and bring a different perspective.

For a religion to become dominant in a specific area, it needs to have been exposed to the religion for long periods of time on a consistent basis, and preferably, for the ruling class of the area to adopt the religion.

Most Bengalis today are Muslim because Bengal had long-term, consistent exposure to Islam, both through conquest from the west, and trade (Bay of Bengal was a major trade hub for the Muslim world). The later dynasties in the Bengal area, though intermittently independent from direct Delhi Sultanate or Mughal rule, maintained this consistent exposure, and their subsequent rulers were Muslim.

For much of India, this wasn't the case. The major Muslim trade routes (silk road and sea-route) never touched most of India, while consistent exposure to Islam (via conquest) was only achieved in the Sindh and Punjab provinces (where most are Muslim today). Much of the rest of India wasn't ever held long enough or securely enough (nominal rule where Muslim dynasties simply collected taxes) for Islam to entrench itself.

With regards to Southeast Asia, I believe the story is similar. Muslim conquest is obviously not a factor here, so the spread of Islam would be reliant on trade. Based on all the maps I've seen of medieval trade-routes in the Indian ocean, the only areas that were consistently controlled/visited by Muslim-merchants were southern-Malaysia and Indonesia. The ruling class in these areas eventually adopted Islam, the subjects followed suit, and the rest is history.

Southeast Asia also includes Myanmar, Thailand, Laos, Cambodia, Vietnam, etc

How does it spread to Indonesia, but barely touch those in SE Asia?

How Islam could spread among the masses of Bengal (without spreading among the masses in current day UP and Bihar states of India) is a question that has now two plausible historical answers

View 1: As the frontiers of cultivation expanded between the sixteenth and the eighteenth centuries in eastern Bengal-away from the core of Brahmanic civilisation-Islam also spread as the "religion of the plough", bringing local people gradually into its fold. This Islamisation did not take place at one stroke, but as a gradual process slowly absorbing the colonisers of the land, who were not yet touched or only just slightly touched by Hinduism. The creation of a Muslim peasantry in Bengal was therefore not the result of any large-scale "conversion", but of gradual incorporation of people residing at the periphery of Brahmanical civilisation.

View 2: Other argue that under the leadership of a group of'"cultural mediators", consisting of a section of Bengali speaking Muslim literati and religious preachers (pirs), Islam in Bengal in the sixteenth-seventeenth centuries acquired a syncretistic face by borrowing generously from local religious and cultural traditions. This reconstructed Islamic great tradition was more acceptable to the masses, as it resolved the problem of dualism between the Persianised and Arabic Islamic high culture of the ashraf (Muslim upper class) and the Bengali culture of the ajlaf peasants.