It's not just the amount. Eating the same amount of calories of pure sugar vs. something like pasta doesn't have the same effect.

If you eat 300 calories worth of a candy bar, then all that sugar gets really quickly absorbed and then the body uses as much of it needs for energy. When it's done using what it needs, it stores the rest of it in the liver and muscle. But the liver and muscle only have so much capacity to store it, and when this 'overflows', the sugar is instead stored as fat around your body. This is why eating large amounts of sugar leads to getting fat, your body doesn't need all of it for energy and excess sugar just gets turned into fat.

If you eat the same amount of calories, but in things like pasta and bread, then your stomach and intestines take longer to break it down, and it is released much more slowly. So your body gets a constant supply of sugar over a longer period of time, meaning there isn't as much excess sugar to get turned into fat. You also don't feel really low on energy and need to eat more soon after.

Of course, eating loads and loads of pasta and bread is still going to get you fat eventually, but it is nowhere near as bad as eating and drinking things that are really high in simple sugars like glucose or fructose.