I used the 1080p public domain tractor sample from
xiph.org/.../y4m and scaled it down for web use while keeping the lossless y4m format. The videos were then both generated with the same command line and the most recent VP8/VP9 encoders as of Jul 26 2013. Though the bitstream was already finalized, both encoders may have been improved in the meantime. Consequently up-to-date encoding results may vary in file size and quality.