Gram for gram, fat takes up more space than muscle

Gram for gram, fat takes up more space than muscle. This means your waist circumference may decrease when you lose fat, even if your weight doesn’t change.

If you’re exercising with weights, increases in arm circumference may mean that you are gaining muscle in your arms (4Trusted Source).

Of course, it is very important to measure the same way each time so that you get more accurate results.

more at  https://www.healthline.com/nutrition/improve-body-composition#section2