The cage is fine- it's well over the mandated cage size and it's not like this pig spends 24 hrs a day in there. Bigger is better but it's not required, just nicer. Since the op mentioned they let their pig out to run around for hours I wouldn't be worried unless the pig is giving signs of stress like cage or fur chewing/hiding all the time or spurt running.
Considering the 4H handbook my sister got when she was showing her hamster said that the basic cage size for a Guinea Pig was 1'x1' (or maybe it was 1'x2'), that isn't saying much.
But, if you look at it from a livestock perspective, I tend to agree with you. Animals that are kept commercially obviously can't be kept in huge enclosures. How you judge the effectiveness of your enclosure is based on the productivity/health of the animal and the behaviors they show. If a Guinea Pig is healthy and isn't showing any behavioral signs of stress/unhappiness...then even if we would like to say otherwise, the cage is fine. Maybe not ideal, maybe not great, but it's fine.
But from a pet perspective, especially if you don't know what behavior/health signs to look out for, it is always best to go above and beyond. If you have the animal for enjoyment/companionship, why not make its enclosure go from "fine" to "super special awesome"? My ferrets would probably do fine with only a hammock in their cage and aspen bedding as substrate. But they definitely much prefer having lots of blankets and beds, and the look of utter contentment on their faces when I gave them a big plushy dog bed the other day was priceless.
I kept a piggy in a 75 gallon aquarium (with screen top) successfully for years. I didn't know she had to have hay and the like (and she was fed a junk diet and was kept on pine bedding), so she ended up passing around four years of age after getting sick. When I adopted Lucy, she was very happy in her 2.5'x4' cage. (She was not so happy in the small cage she came to me in.) After Lucy passed and I adopted Louie and Dot, they were in the same 2.5'x4' cage, but when I was no longer able to give them out of cage time every day (which Lucy almost never had), Louie got cage stressed. That was one of the reasons I ended up rehoming them. They went to a family that had a 10.5sq. foot cage for them (C&C) and planned on giving them out of cage time on top of that.