Higher carbohydrate intake increases muscle glycogen storage and associated water retention, leading to increased lean mass measurements and perceived muscle fullness, while lower carbohydrate intake reduces these effects.

Study: Carbohydrate refeeding after a high-fat diet rapidly reverses the adaptive increase in human skeletal muscle PDH kinase activity.

When you eat fewer carbs, your muscles slow down how they use them; when you eat carbs again, your muscles quickly turn back on their ability to store carbs — which means more glycogen and water, making muscles look fuller and heavier.

Study: Human skeletal muscle pyruvate dehydrogenase kinase activity increases after a low-carbohydrate diet.

When people eat very few carbs, their muscles stop using sugar as fuel, which means they store less sugar (glycogen) and water — making muscles look and feel less full. This study shows exactly that happening at the enzyme level.

Study: Muscle glycogen storage after prolonged exercise: effect of the glycemic index of carbohydrate feedings.

Eating carbs that digest quickly after exercise helps muscles store more glycogen, which also pulls in more water, making muscles look fuller and heavier. Eating slower-digesting carbs doesn’t do as well.

Study: Comparison of three type of muscle glycogen loading interventions using a very-high-carbohydrate diet in an elite male racewalker: a case report

Eating a lot more carbs made this athlete’s muscles store way more energy and water, making him 1.8 kg heavier and feel bloated — exactly what the claim says happens when you eat more carbs.