Bullet

If your time runs out and your opponent at least has a pawn -- then theoretically that pawn can become a Queen and mate you !    So your opponent is creditted with a win !

Best to play with a time control that gives you additional seconds when you complete each move.   This way if you are a ridiculous amount of material ahead, you can still comfortably win -- rather then lose on time

Material imbalance almost always does not matter in such scenarios. "9 points up" is nothing compared to the "absolute loss of time".

The side still having the time must, however, have at least a surviving pawn or rook to win the game on time.

Doesn't matter if it's bullet, blitz, rapid or classical, the goal of the game is to checkmate, not to have more points. For example, in this diagram I created loosely based on Paul Morphy's legendary opera immortal game (I added a few queens just to prove a point), white has a lot less points than black, but the rook goes to d8 and it's checkmate and white wins even though it has a lot less points.  As mentioned above, if your opponent has enough material to checkmate you and you run out of time, they win, even if all they have is a king and a pawn, since that pawn can promote to queen and checkmate you =]

Also, if your goal is to improve you shouldn't be playing bullet or blitz. If you would like to get better you have to play longer time formats, such as rapid, so you have time to think about your moves and consequences of making such moves.