I gather you don't understand "gauging ". If I am wrong, I apologize. When we describe a gun as a 28 gauge, that means that 28 balls, the diameter of the bore, will weigh one pound. They are approx. .550 in diameter. A "20 Gauge " barrel means that only 20 balls,the diameter of the bore, weigh one pound. The Bore diameter of a 20 gauge is approx. .615". A 14 gauge is .693" bore. The 16 gauge is .662" diameter. The 24 gauge is .579" bore.
Corresponding calibers( for rifled guns) would be .54 caliber for the 28 gauge; .58 caliber for the 24 gauge; .62 caliber for the 20 gauge; .66 caliber for a 16 gauge; and .69 caliber for the 14 gauge.
In modern gun manufacturing, its common for gunmakers to bore the same OUTSIDE DIAMETER barrel stock for both .410 Bore, and for .28 gauge barrels. In Current ML smoothbore practice, its common for barrel makers to use the same outside diameter barrel stock for both the 20 gauge, and the 28 gauge. Only rarely will you even see a 24 gauge ML shotgun, and they are usually found in Europe.
When the larger diameter barrel stock is used to make a smaller gauge barrel, the walls of the barrel are thicker, and the finished barrel will weigh MORE than when that same stock is used to make the larger gauge barrel.
Its not uncommon to find a 28 gauge half stock fowler that weighs in the 5 lb range, and is very fast to swing on flushing game. But, the barrel has to be made from smaller diameter barrel stock, if the gun is to be that light.