I don't think that's correct. The overhead on the side with two seats is shorter because it's over 2 seats instead of 3. The same number of bags can fit side by side, they just can't be as long. That's why the FA's will usually tell people with rollaboards to use the 3 seat side and use the 2 seat side for smaller bags.
The bin is not as
deep, but it isn't
shorter (or narrower, depending on how you look at it). I just flew on this type of plane, and yes, you can fit two rolling bags end to end. That is what the flight attendant told us to do, and it worked fine. On the other side, the bin was the regular depth and so they went in wheels first (three of them).
It CAN'T work out unless you have a bin for every row. IIRC, each "bin" spans ~two rows. So on the 3 seat side, you have one bin that will take three bags, but it has to service 6 seats (two rows).
Now, if someone doesn't have a bag to put into the upper bin, THEN things can work out.
I didn't say there was room for every carry-on, just that it works out the same as any other airplane. It's true that the left side of the airplane (facing down the aisle) only holds two, but there are fewer seats on that side.
Proportionally you have the same number of seats per "rolling bag space" in the overhead bins on an MD 80 as you do on a bigger plane, such as a 727. The ratio is still about 2 to 1, depending upon how close they decide to put their rows. And of course, on a really big plane like an older 747, there might be even less because of all the seats in the middle. The fact is that most people (especially leisure travelers) don't take rolling carry-on bags, which is great for people like my family who
only take a rollie, and have for years and years due to the convenience. However, more and more people are going this route because of baggage fees, and now we're seeing gate-checking on bigger planes where we never used to see it.
On regionals, of course, (like the Canadairs) we always have to gate check because they're little planes with little overhead bins.