They are totally fine and are all made in accordance with the contest requirements. 
The source here is a 1966 movie, so it's rather naive to assume that you can get the same result as if you were using some recent cartoons, the good HD quality source files of which can be easily found. 
I used a good quality source file with high resolution and bitrate. And in my opinion, it was still not good enough to get 848px width gifs. But suggested widths were 848, 640 or 380. To me, 380px width was the best choice for those gifs to preserve the picture quality and to get a minimal file size in the end as an extra bonus. 
And you probably watched them on desktop, where for some reason the enlarged versions are played. Had you watched them on a smartphone or a tablet, they would've been perfectly fine)

You do know that one exact emoji can express different emotions. Or may I say even one emotion that can be expressed differently. Just to give you an idea, this one 👏 can express literally applause, or it could be a demonstration of respect, or of joyfulness and gratitude (like in my 18th gif), or it could even be a sarcastic clap (like in my 83rd gif). Saying that, I mean that mind blown is not always when your head is exploding and your brain is out like some ground meat. Apart from anger and annoyance, it also can be a shock out of surprise or admiration, for example. Of course, I thought about choosing a party for that one, but decided to go for mind blown because it's quite difficult to find something beautiful but not nasty for this particular gif. As for №2, some people make this move to express their coolness) I don't know how it is called, but I think it correlates with the mic drop move which is nothing but cool.