A 30-pound dumbbell is a direct labeled-weight comparison; adjustable dumbbells are sold with settings up to 30 pounds. For example, MX Select's MX30 dumbbell can be set to 30 lb. You can also make the same total by combining three 10-pound weights or six 5-pound weights.
These comparisons describe weight, not a fixed size. A 30-pound object can be compact and dense or large and bulky.