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.
In an explicit flashback. the person is involuntarily transported back in time. To the person, it does not seem so. What they experience is being experienced as if it were happening in the present. An explicit flashback involves feelings and facts. Flashbacks from early childhood are different. They do not include factual information. Until about five years of age, factual - or explicit - memory is immature. But implicit memory, the memory of an emotional state, may go back to birth.