Ok, I did a test to see what 15% of original size represents in real world pixels. I took the frame sizes 0f 867px by 694 and reduced it to 15% of that size. This makes a sprite just 72 pixels high. At 72 pixels high this is precisely what you would expect. I think with the heavy amount of reduction you applied it turned out rather well considering.
You won't ever be able to reduce an image by so much and maintain it's original resolution it doesn;t matter what program you use. If you reduce an image to 72 pixels you get precisely 72 pixels of information. All the original information of the high resolution format has to be striped out and turned into a representation in just 72 pixels. Imagine if we reduce it to a 10px by 10 px image? It would be impossible, you just couldn't build a detailed representation of this sprite in 10x10 pixels.
I think you either need to find the lowest possible limit to get the resolution you want, perhaps 30% or if it's essential to render sprites so small then perhaps look into a pure pixel art route.