My experience is similar in that weight seems like the overriding factor rather than dimensions. I suppose a much larger bag would make you susceptible to a more thorough check.

I am extremely surprised that many bags that claim to "respect" common airline limits of 158cm total do not include handles and wheels. Both the stores and the manufacturers say they pass but I suppose part of it is the push for sales. I haven't heard many stories of these "borderline" suitcases being flagged but someone once remarked that they would print a copy of the specs from the manufacturer and place in permanently in the bag just in case.

I see larger bags 29inch and above, that are common amongst family travellers from middle and developing countries a lot more often. I am guessing most do not travel frequently nor are efficient packers. Maybe perhaps they like to have that extra space for shopping. It certainly is a very viable market for any manufacturer to cater to this growing class.