So if a story is excellent, readers will get angry that they overpaid by a buck and didn't get, say, another 10k words out of it? I'm not being sarcastic, I really am amazed by this and I have very little experience with ebooks.
Would the opposite inspire loyalty?
What Steerpike said.
Imagine you're barricading yourself in the sofa with a good book and it's finished before even half the candle's burned. Even if the story was great it didn't last as long as you'd expected.
I don't think the same really applies if it's too long. If the story just drags on your excitement will slowly fizzle out, while if it's too short your disappointment is more immediate.