mb and Mb mean the same thing (to be exact mb is meaningless, but it is understood to mean Mb), just like kb should be written Kb, and gb should be written Gb. But since kb doesn't mean anything otherwise, then it is obvious. But if you say "1mb=8mb" then it is very confusing, so you should be careful when dealing with the 'B' vs 'b'
The M is the prefix: Mega-Byte or Mega-bit. So Mb and mb mean the same thing, and MB and mB mean the same thing! So when you say "actually megabyte is MB or Mb" that is incorrect, MB is Mega-Byte, Mb is Mega-bit (you can remember it easily because the B looks like an 8, so the big B is 8 times bigger than the little b, which is a good way to remember there is 8 bits in a Byte).
Anyway I don't use origin so I can't compare, like I said earlier I don't want a second program to do what Steam already does fine.