GIMP :: Why Image With Most Of Pixels Removed Is Creating A Larger File Size

I've been using GIMP to make GIFS for a long time now, but recently the "optimize for difference" option I've used is having the reverse effect it should have- ie, it's making the file larger. Here's an example. It's the exact same image, same dimensions, same amount of colors. The first one is unoptimized, each frame is a full picture, and it's file size it ~149kb. The second one is the same as the first, except optimized with about ~50% of the pixels removed from each frame (except the first frame obviously), and yet it's file size is ~155kb. I can not for the life of me figure out why an image with most of it's pixels removed is creating a larger file size. [URL]...