So, I recently downloaded Paint Tool SAI on my Surface Pro from an online downloading service. When I wanted to test out the brushes, I noticed that, the smaller my brush size got, the more pixelated the brushes got. How can I fix this?
After studying some tutorials and watching some videos, I've found out that the simplest way to smooth the brushes even on the Paint Tool SAI is to change the Smoothing factor. It's a tiny option that has a field to modify the number when you access the brush tools. It's located under the main menu bar on the right side.
Increase that number to 15, for example and then draw the brushes and you'll see it won't look pixelated anymore.