I am not being ridicuclous. It isn't the size of the fish, it's the swimming room and activity that is also taken into account. Sure you could fill your tank with whatever fish you want, and have adequate filtration to keep the water clean, but would the fish be happy? No. Would it be humane? No. So that is why those fish are not suitable for your tank.
+1 for this. A happy fish is a healthy fish.