Some story lengths are very hard to sell.
And while a story can be expanded, with care, by not just adding episodes but by adding complexity -- a new character, perhaps, or a bit character promoted to someone for whom the main character's plot becomes deeply significant -- it's a lot harder to lop out parts of a larger story. For one thing, the expansion trick only works if you marry the new subplot so deeply with the pre-existing story that it's hard to see where the joins are. To lop off the chunks requires seeing joins you were at pains to hide, or perhaps never noticed.
Naturally, of the two stories foremost in my mind, it's the one that's already noticably long that's hinting that it needs a lot more development in sections, not the one that could absorb quite a bit without reaching the Unpublishable Void length