For me using a big brush size is the best for filling in large areas. From the image in your gallery it looks like you are filling in on the same layer as your line art, which would make it really hard to do that without painting over your lines, and could be the reason for you using small brushes? Put your line drawing on a different layer as your fill if you can (I haven't used Essentials so I'm not sure if you can use layers), that should help.
I'm not sure what kind of brush you are using, but it looks like some kind of water color brush. Those can be hard to use because layering makes the color darker. Try using the acrylic or oil brush to do fills so you get a solid color.
But yeah, let us know more about the process you are using to create these.