Champion sizes

So my girlfriend is currently playing as Annie. and I noticed something weird. Sion is quite big, as we know it, but for a while, he looked HUGE and I mean Lulu ult huge. What is it that causes these growths? I know Lulu's ult causes champs to be bigger. As well as chogath's ult causes him to be bigger. but what else causes a champion to scale in size? Is it because annie is a tiny champ so it makes others look bigger? Is there anything else to make champions bigger in scale?