What is the difference between mass market paperback and the regular paperbacks. One is bigger????

A regular paperback is sometimes called a trade paperback. It's essentially the larger size book. These come in a variety of sizes, but all taller and wider than the mass market size. It often, but not always, has the same pagination as the hardcover edition so the publisher doesn't have to do as much formatting to release this version. The mass market paperback is the smaller one, though, about 6 3/4 inches or so in height. The publisher has to reformat to get the text from the HC to fit into this size.  There are now also the taller size MMPB, 7 1/2 inches.  I don't have any idea what the criteria is for why they make some of them taller. I have series that have both heights so it's not consistent. Or maybe they publish in both sizes and since PBS doesn't distinguish there's no way to screen unless you use an RC.

A regular paperback could be a "Mass Market" paperback or a "Tall" paperback or a "Trade Size" paperback.

A "Mass Market" paperback is the smallest of all three. About 6 1/2 " tall x 4 1/4 " wide.

I've always thought of the mass market PBs as  the 'regular' one because they were the first size available.

For this site, the term "paperback" includes anything bound with a paper binding, regardless of size. Mass market are the size you find in a grocery store aisle or an airport kiosk. Trade are larger -- closer to the size of a hardcover, as stated above. Generally, trade paperbacks are on better quality paper than mass market paperbacks.

Generally, publishers DO use different ISBNs for the different editions, but don't count on it.  We often find MMPs released on the same ISBN as the TPB or even just a better quality smaller paperback.  On rare occasions, they have even used the same ISBN as the HB.  Very frustrating...