(No spoilers here.) Kingofooo.tumblr.com, the "Official Adventure Time Crew Art Tumblr" posted this interesting tidbit a few days before "Something Big" aired, regarding how the episode came about.

Mid Season 5 we tried to make a 45 minute Adventure Time TV movie. We almost did it, but the thing was a mess and needed so much work to get in shape. All the individual parts were really cool, but they weren't hanging together right, and the end seemed impossible to figure out.

...

I was really psyched on my section and started to feel certain this movie was never gonna see the light of day. So we started pushing that we should cannibalize the parts and make them into episodes. That's basically what happened. I got my opening section and used about half of it of it to make this episode a standalone story.

- Jesse Moynihan Writer/Storyboard Artist

Very interesting indeed. If this was, in fact, just the opening portion of a pitched Adventure Time movie, I wonder if that means some of the very obvious hooks for future episodes that the last episode ended with would have been what the rest of the movie revolved around.

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While I loved the episode, there was a very clearly (and I thought deliberately) uneven tone/pacing with the ending, and the episode honestly kind of makes sense as the opening of a larger story.

In any case, you can read more about what I thought about "Something Big," with spoilers galore, here or here.