It is appeared that these radical extremist hindu preacher and politicians able to sold this myth of 20 million illegal Bangladesh well to these ill informed and less matured lot quite easily. But what they does not understand is that 20 million means 1500 people crossed the border each day for the last 40 years. N if that really happened there should have been a huge impact on the total population of Bangladesh. But if we see we will find that population of Bangladesh always increased steadily, never fall down.
These low IQ people even does not have brain to see the reality from the demography of Bangladesh. 20 million is almost 13% of the total population of Bangladesh. If it really would have gone to India then there would be a huge short fall of population that never happened.
These illegal migrant story is usually raised by radical hindu politicians during election time to discriminate against the minority population who are mostly muslims. They do it mostly in East India where these radical group has low popularity base.
Population of Bangladesh since 1970. Now here there is mark of reduction of population.
Human Development Indicators - Google Public Data Explorer