Most of our planes have pretty big overhead compartments. The DC9s all have the large overhead bins that can accomodate a roller-bag with the handle facing in and the wheels out (on the three-seat side). This allows two roller-bags to be put in the same compartment.

The 727s only have small bins. They're being retired next year so they were not fitted with the new bins/interiors.

The newer 757s also have the larger bins on both sides of the aircraft, though there are some that still have the small bins that only take roller-bags sideways.

The DC10s and 747s have large bins, too.

I guess more care should be taken in measuring up the size of some of these carry-ons. I've seen some garment bags stuffed to the gills that they don't even fit on TOP of the sizer boxes!

I must say, however, that people are carrying less on board these days. It seems that they are more more trusting that NW will get their bags to their destination with them. (After all, isn't that what's supposed to happen!?)