Does this happen with all images or just this one? If it's all, then that sounds like something is wrong with Imagick. (Note that I don't know where the image you're referring to is.)
However, animated images are effectively much bigger than their potential file size. To resize them and maintain animation, they have to be decompressed, each frame manipulated, and then recompressed. You could have a small image that actually has a lot of frames, which means a lot of work to resize.