It does depend, very strongly, on the department for which you're applying. Graduate school is different from undergrad in that you're only applying for a specialized area of research, so asking questions about grad school only makes sense in the context of a particular graduate program at MIT.
It's generally easier to get into a master's program than a PhD program, at MIT or at any other top science or engineering graduate program. But the numbers can be misleading -- the aero/astro grad program admits about 50% of applicants overall, but a huge number of those admitted are former MIT undergrads, and almost 100% of MIT undergrad applicants are admitted (my husband's year, only two were rejected).
I would not use the fact that 19.7% of applicants to MIT's grad programs are admitted to argue that admission to MIT's grad programs is easier than admission to the MIT undergrad program. For one thing, a very large number of MIT grad students were themselves MIT undergrads (about 20% of each graduating MIT class goes on to grad school at MIT). For another, admission to top science and engineering grad programs in general relies on the applicant doing a number of things right during undergrad: pursuing interesting and meaningful undergraduate research and getting glowing personal recommendation letters from professors, to name two. Merely getting a 4.0 as an undergraduate and a good score on the GRE does not by itself make an applicant competitive in the least for top science/engineering graduate programs.
Is there anything I can clarify?