I'll start at the end, a big win shouldn't start at 20x. In most games that have a buy bonus that requires a 100x bet the special screen that accounts for 20% of the bet is a joke.
A big win is at least 100x in my opinion, because that makes cashout possible in most cases. In most traditional games or scratch cards it is a 3rd tier prize, which is a pretty good return on your purchase and in casinos it appears quite often.
500x is at least a huge win which in most cases guarantees a cashout, above that it is known, a livechanging win.