The grind is real. This only counts pixels you painted/colored - not pixels you filled with the fill tool. It counts pixels painted in both Pictures and Subscribed Items. If you use the fill tool on a subscribed item it will not count towards the completed pictures of a given size but any pixels you painted on that picture will still count towards this goal
A tip - if you have a big block of the same color you can hold down the mouse cursor and use the wasd keys to move side to side and up and down. The farther zoomed out you are the faster the mouse will go using this method.
Another tip - the easiest way to see how close to this goal you are is to go to View My Achievements and then check out the leaderboards. The only thing the leaderboard tracks is the number of pixels you've painted/colored.