I collected all the seasonal Halloween decorations (October 2022) and it didn’t give me the large..

To get the grand prize, you must collect the necessary points, not have one of each item. Each decoration in the set has designated points. You can mix and match the items, as long as you collect the required points. And you must have construction completed and ribbons unwrapped before the deadline. When you tap on the seasonal set icon from your island, there is a two-toned bar that shows your progress toward the grand prize.

Specifically for the Spooky Surroundings Set from Oct 2022, if you had collected one of each item, you would have earned 2350 points (jack o' lanterns), but you needed to have 2900 points (jack o' lanterns) for the grand prize, the Gabled Ghost House. So you would have been 550 points short, and would not have received the grand prize. Note that some monthly sets, if you buy one of each item, you could reach the required points.

If you had collected the required points, but still did not receive the grand prize, then their may have been a glitch. Contact customer support through the "help" button in the game settings.