Percentage of Muslim population was lesser in Bengal Presidency because it was including of large parts of Hindu majority areas,
You are true in this statement.Then Bengal presidency included hindu majority Bihar,orissa,assam.After their shedding in 1911, Remaining Bengal became muslim majority.
Hindu population in the region now known as Bangladesh has decreased steadily since 1940's primarily because of the exodus due to religious persecution, the very large population of East Bengali Hindus in India didn't fall from the sky.
This statement is only partially true.Some hindu left BD indeed for persecution.But most of them left for supposed better prospect in India.Urbanization is an ongoing trend in both India and BD.Many rural hindu from BD prefer to settle in Kolkata rather than Dhaka,Chittagong as it very easy for them to get citizenship.Hindu in general see India as their real spiritual homeland where they can prosper unlike Bangladesh.They think India as a big wide welcoming place to settle comfortably.There are many cases in Bangladesh where hindu family buy property in kolkata gradually with income in BD ,send their children in Indian school and one day move over there after selling their possession in BD. Is this sound like persecution to you?
You may ask why opposite is not true? Why indian muslim don't migrate in BD? I think firstly it may be because Bangladesh,with indepence in 1971,no longer believe in Pakistan ideology,which is muslim homeland for south asian muslim.So after 1971, door is shut for indian muslim.Muslim migration in BD happened during 1947-1971 period.Bihari muslims are the testament of that.
Secondly, 80 percent Indian muslim have no cultural affinity with Bangladesh and they live far of from Bangladesh.Other 20 percent Bengali muslim live near Bangladesh should be the natural candidate for migration.But they are not migrating BD.I think reasons may be due to the fact that, Indian bengali muslim lives in muslim dominated districts where they are majority,so less minority feeling and incentive to look elsewhere.
Another reason is that ,up until few years ago,west bengal was economically better off than BD.So it have influenced both Bangladeshi hindu and west bengal muslim,encouraging the former and discouraging the letter for migration.Another plus point was Kolkata which was largest city in south asia was a magnet for both west bengali people (hindu and muslim alike) and Bangladeshi hindus.While Dhaka became big only recently.In 1950s and 60s west Bengal was much more urbanized and had vibrant city life compared to then BD and also economically west bengal was easily twice as rich then east pakistan on per capita basis.
So hindu migration is more about psychological affinity and cost benefit calculation than any physical danger.I am also telling this others like
@Nilgiri who may have curiosity on this issue.