The general logic of Inflation is that some field popped into existence just long enough to flatten out the universe, then disappeared again. Before the field, the universe had tiny fluctuations in the plasma. Inflation blew these up from the size of an atom to the size of a grapefruit (If I understand the scale correctly). What I don't understand is: why was inflation needed? Why wouldn't these fluctuations grow in a normally expanding Big Bang scenario?