A cup of plain flour weighs roughly 120 grams. A cup of white sugar weighs roughly 200 grams. The cup did not change size: sugar is denser and packs differently, so the same volume carries two thirds more mass.
That is the whole problem with volume measurement in baking. A recipe needs a ratio of masses, and a cup measures space.
The packing problem
Sugar is granular and pours to a fairly consistent density. Flour is not. Scooping the cup directly into the bag compresses the flour; spooning it in loosely and levelling the top does not. The gap between a packed cup and a sifted one is large enough to change the texture of the result.
This is why recipes that take volume seriously specify the method, with phrases like spoon and level, or sifted then measured. The instruction is there because the measurement is unreliable without it.
Why it matters more in baking than in cooking
A stew tolerates an extra ten per cent of carrot. Bread and cake do not tolerate an extra ten per cent of flour, because baking is a set of ratios: flour to liquid, flour to fat, flour to raising agent. Change one term and every ratio moves.
Professional recipes are written as baker's percentages for exactly this reason. Everything is expressed relative to the flour, so a dough with 1,000 grams of flour and 670 grams of water is at 67 per cent hydration whether you are making one loaf or forty. The baker's percentage calculator works in those terms directly.
Scaling is where volume really falls apart
Doubling a recipe in grams is arithmetic. Doubling it in cups means doubling a measurement that already carried an error, and the error doubles with it. Halving is worse, since half of three quarters of a cup is an awkward quantity most kitchens cannot measure accurately.
Weight has no such problem. Scales read the same whether the number is 37 grams or 1,480.
Which cup, anyway
There is also no single cup. A US cup is about 237 millilitres, a metric or Australian cup is 250, and an imperial cup is different again. A recipe that says "one cup" without saying whose cup has already introduced a five per cent uncertainty before any packing question arises, which is the same trap covered in the piece on units that share a name.
Converting a recipe once, at the start, removes all of this. The grams to cups calculator converts using typical densities for common ingredients, and the cup to mL converter asks which cup standard you mean rather than assuming one. Both are most useful for translating a recipe into grams once, after which the scales do the work.