In Understanding Comics, McCloud made the claim that manga supposedly uses more circuitous storytelling because of the formats of their books.
In my paper on Japanese VL, I dismiss this on the grounds that nothing about longer formats gives people the drive to make slower paced narrative. Just because you have ample space doesn't mean you're going to use it to let the story linger more. You could use that space to fill in even more "compressed" storytelling.
Thinking more about this, webcomics are another good example against this theory: from my knowledge, we haven't seen a vast decompression of storytelling on the web due to the completely unrestricted "infinite" space allowing authors to freely use (though feel free to prove me wrong!). On the one hand, you could say that they aren't effectively using the space that they have at their disposal. However, the other side could say that they're using it to achieve just what they want: they have no restrictions, so what they're producing is entirely their preference.
Personally, I think that there are numerous explanations for what might be going on in manga storytelling. Here's a few, some of which were in my paper...
1) It's just an inherent part of the difference between Euro-American VLs and Japanese VLs. We don't expect spoken languages to be the same, why should visual languages? Could "decompression" simply be a result of the development of how JVL evolved?
2) They're using the VL as a language: Manga use less text than American and European books. With more reliance on visual modality over the written requires it to take on more expressive weight. The result is more complex structure in the visual sequences. This is comparable to studies asking people to only gesture with no speaking. The result is something that looks closer to patterns like in sign languages (though still not SL).
3) The cross-cultural differences focusing on environment over action requires more space devoted to "setting a scene." Research seems to suggest that Asian minds are more interested in the broader environment than the specifics and individuating different elements of the environment take up more panel space than simply presenting it as a whole, backgrounded to the actions.
Notice that in all of these cases, formatting is entirely secondary. Indeed, it's somewhat interesting to think that formatting is one of McCloud's explanations, because much of his work is about transcending formatting. Here, the explanations focus on cognitive reasoning — meaning we should see the effects no matter what the format.