The Blu-ray of this series carried over heavy noise from Fate series, but seems didn't inherit abundant funding like Fate had. The native resolution is only 720p with some scenes even lower than that. The heavy noise prevents artifacts such as colour-banding, but leads to high bit-rate and lack of sharpness. High speed moving scenes also have aliasing due to low native resolution; it is hard to notice during normal watching, and forcibly using anti-aliasing would easily damage other normal scenes such as snowflakes. After due consideration, anti-aliasing was abandoned.
This is pretty much the lowest limit of the file size. I would recommend downloading the 720p version because the source was up-scaled from this resolution, and we can descale 1080p back to 720p almost losslessly.