The McDonnell Douglas DC-10 and Lockheed L-1011 are wide body "heavy" aircraft with all door exits, no overwing window exits. Mid fuselage doors were built into these aircraft to provide multiple points of entry and exit to speed up normal passenger boarding and deplaning, not specifically for emergency exit efficiency. Positioning them near mid-fuselage also enables more rapid emergency evacuation as a side benefit.
Boeing 737 and Airbus 320 aircraft are traditional single-aisle designs with doors only at each end, and supplemental window exits provided over the wings. The overwing exits are not normal passenger entry points and are sized to fit the fuselage above the wing where a door would be too large and weaken the fuselage structure too much at that location. When the FAA certifies the aircraft for emergency exit procedures, it takes into account the fact that the windows will not be able to accommodate the same number of passengers as the doors. More recent aircraft models have improved the over wing window exits by removing one seat from that row and providing more space in the row that serves as the exit lane.
In addition to not being as large and accommodating as the main door exits, the window exits also rely on passengers to operate the exit in the first place as opposed to trained flight attendants being seated next to the door exits. When in flight attendant training, we were instructed how to open the window exits, and I can tell you they are a lot heavier that you might think, and there isn't a good place to stow them either. But in short, the FAA knows these exits are not as efficient and may not be as reliable as the door exits. Nevertheless the aircraft manufacturers have demonstrated the sum of all the exits are sufficient for the required evacuation time in an emergency.