One colour areas are done using copies as well, the engine doesn't scan for this. However, as only the edge blocks of these areas need to be constructed, it is not a big deal. I can image that I could have scanned for that as well, but it occurs only very seldom, so maybe testing for it might even be relatively expensive on average. In Core Dump the black bars between the camera windows are fills though

Sprites are plotted as cells, and they have to be, because cells are re-used among animation frames. Say you have a tank with turning wheels, then each animation frame has largely the same set of cells (blocks) except for the wheels. In particular, the big spider thing in Akin shares many blocks between its animation frames. So in general, big copies are not an option, as this would mean that the sprites take up much more VRAM (as lots of unnecessary information is copied throughout the animation frames)

Also (I forgot this in the previous post), although big copies are in general faster than multiple small ones, this is not the case here, because we can optimize stuff on the small blocks. Consider that big spider again: for the blocks that make up the bulk of its body, which have no transparent pixels, we only need to do high-speed copies, and never have to construct the background under them. If you would want to use a big copy, you would have to use a logical copy for the whole spider, and still have to reconstruct all the background under it.

So because of local optimizations of the blocks, and re-use of blocks throughout the animation frames, many small copies are much more efficient than big ones.