check your file>publish settings>image settings and experiment with the settings.
(my guess is you're exporting as a texture 32 bit to see that increase.)
Optimizing PNGs before import is pointless, because Animate re-compresses them when it publishes. If you want them optimized, you'll have to optimize the published files.
unless its a PNG8 you can't crunch PNG's at all. And if you are using a PNG8, try using a GIF instead ... seems they are usually smaller, If you are using a PNG24 and taking advanrtage of alpha transparency then the file size is was it is.
But in Animate, if you double click the PNG in the library, then you can spot reduce and set a custom setting for this problematic PNG. Also make sure the height and width is as small as it can be... since the large the file in size, the larger the optimized file is too.
Hope this helps.