If you used a lot of transparent av, its wise to set matte to none, or else, you get some white specks in the transparent area.
Another manual choice to drop the size is use the dithering amount, even the automate option used different dithering amount.
Another one is the lossy option, if you are short of 1 or 2 Kb from your intended size (50Kb) use lossy option to tone down the filesize, you lost a bit of quality, but if you end up using higher amount then 10 for lossy, try something else, keep lossy amount as the last option to trim down filesize
