I haven't travelled down the back of a domestic flight for a while, but I was on a 73 last week and every overhead bin was full of small carry-on luggage. There seems to be a rise in carry on baggage for two reasons, 1. the length of time it takes to retieve checked baggage at the other end, and 2. the extra cost of checked baggage. The NTSB has noted a fault with overhead bins that may cause injury in survivable accidents.
Have a look when you next jump on board a flight and check this outthis NTSB hyperlink for mods to 737's
http://r20.rs6.net/tn.jsp?et=1109391...E-I2lHmS2IT3I=