Doesn't matter for all ideas. If you have a bright idea for a comic little interlude, it can fit as a piece of comic relief in a much longer work. The trouble arises when you have a structural idea, that determines much of the story.
Particularly with climax. A funny little climax, or a twist, can work with a short story, because it only has to bear the weight of the short story. To stick it on a novel, with all the complexity of that a novel needs, would be to make it anticlimactic. This is why a Feghoot is not only always a short story, but always a short-short; you can't have an entire novel leading up to a bad pun.
I wrestled with some ideas before I got the notion to put them little short-shorts, told in the character's voice. The endings weren't strong enough to carry a longer story.
Sometimes this is the problem with padded novels. It's not just that the padding wasn't integrated with the story. It's the story really couldn't support a novel of this length, no matter how you integrated the stuff.