Compare the 12ga 1 oz load with a 28ga 3/4 oz load. 3/4 oz is 60% of 1 oz. 30 yards is 60% of 50 yards. "IF" however you think the 3/4 oz load is equal at only 30yds in comparison to the 1 oz load at 50yds you are badly mistaken. The comparison is more on the order of being proportional to the √ of the shot charges. Thus √3/4 √1 1/4 = 77%. The 3/4oz load will essentially do anything at 40 yds that 1 oz will at 50 yds.
it is extremely easy to get a shotgun which will kill anything you properly point it at at 30 yds. As the range is extended a lot has to be added for every increment of increased range. The further you extend it the more that has to be added for each increment. As range increases the shot size has to be increased to give adequate penetration. This results in thinner patterns so the charge weight or choke has to be increased to ensure adequate pattern.
I am well aware that when adequate sized shot was used occasional "Lucky" kills have been made by one pellet hitting a vital spot & also that the absolute center core of the pattern will kill beyond what the overall pattern will. Be honest though can does anyone here feel reasonably comfortable that they can get 1 shot in a vital zone or always put that absolute lute center core of the pattern on a fast moving bird often enough to greatly extend the range of the shotgun. I am well aware that I Cannot. Inside of about 35 yds there is really not that much to choose from between a light loaded 12, a 16, a 20 or a 28.
Although I do not own one a 5 lb 28 gauge firing 3/4 oz loads would be a joy to carry & would cover the vast majority of my needs. I have absolutely no desire for a 28 weighing more than about 5 lbs nor firing more than 3/4 oz of shot. If it goes beyond that I'll just take a bigger hole down the barrel.
