I assure you both, #7.5 or #8 shot is perfectly fine for a defense load. In your residence, take a look around and see what might be the maximum shot distance you have. Unless you have an extraordinarily open floorplan, you are likely looking at the maximum range to an attacker being around 20 feet. Regadless of the load, at 20 feet a 12 gauge round will hit a human as essentially a solid mass of lead.
Penetration is a concern for many people that actually have to fire the weapon, especially those with families. You should always be concerned with what’s on the other side of the bad guy.
Those who doubt this should go to the range and, if practical, place a heavy coat, several t-shirts, and some ballistics gel at seven yards (21 feet). You might get laughed at as you’ll be aiming your shotgun at something that’s extremely close. Fire the weapon and check both the dispersion and the penetration on the ballistics gel. If you like, measure the width of a human from the side. See what you think.
This argument is way older than the web itself, but it’s been argued ad nauseam since its inception. I can assure you of that.