Optimization is done by managing the amount of frames your animations contain - no duplicate frames & by using the least amount of frames as possible. It's all about finding the right balance between the amount of frames & the delay/pause value between each frame. Remember it's 2D, not 3D, & it's not an animated cartoon/movie, so it doesn't need to look ultra smooth.
Other optimization is done by cropping as much canvas space as possible from the animations/images. Large canvas = larger file sizes.
Also, your characters should be approx 50% (give or take) height-wise of your default game resolution when they are scaled up to 100%. Creating characters that are the same height or more than the default game resolution you specified will be massive when it comes to file size.
Last but not least. Webp really makes a massive difference. Webp (lossless) will be up to 1/3 smaller in file size than png, & webp (lossy 100%) will be up to 1/2 smaller in file size than png. Smaller file sizes = less hdd space required & faster loading times.
In regards to the webp export option in the build section of the editor, I have no idea if it's any good. I personally convert from png to webp & import the webp files while I work on a project. I use a free batch image converter called XnConvert - you can find out more about it here