I have to admit, I didn't have time to read each post fully, but skimmed through, so this point may have already been made. Another issue is water displacement.
Mine is advertised as 4-5 people, it is a 6' dia. Bullfrog 151R. I've had 4 people in comfortably and there is a spot between some jets that can act as a seat that gets some water action from adjacent jets, it's one of my favorite locations actually, yet if 5 larger people used it at once, the water level would have to start off below the fill line to stay in the tub. After having 4 average size adults in the tub one night starting with the water at the fill line, I had to add water the next day to make up for splash out.