In 2015, Google has promised free unlimited photos and videos backup from our phone as long as we allow Google to compress some of the data in our media.

Looks like there are some changes as of 6th December where Google changed some supported video formats. Now it will only backup video that at least 1 second long and only these file formats;

• .mp4
• .mod
• .mmv

The unsupported videos that uploaded after 6th December will take up your account storage space.

Source: Google