MB is larger. There are 1024 KB in a MB. MB are bigger, just like GB are bigger than MB. Just think of Kilo being small, Mega being large, and Giga being huge, and then there is TB, but I won't get into that right now.
The word explicit means clear and fully expressed. If something is explicit, there is no question as to what it means, no hidden connotations and no room for misunderstanding. If a fact is explicit in a reading comprehension passage, it is stated outright. For example, if the first sentence of a story is It was a dark and stormy night, that is an explicit fact.
The origin of implicit is found in the late 16th century. In the same manner, the origin of explicit is found in the early 17th century. Moreover, explicitly and explicitness are derivatives of the word explicit. On the other hand, implicitness is a derivative of the word implicit.