Google+ uses machine learning to provide up to 75% bandwith savings per image served

RAISR, which was introduced in November, uses machine learning to produce great quality versions of low-resolution images, allowing you to see beautiful photos as the photographers intended them to be seen. By using RAISR to display some of the large images on Google+, we’ve been able to use up to 75 percent less bandwidth per image we’ve applied it to.

This is currently rolled out to a subset of Android devices. It's being applied to more than 1 billion high-resolution images per week, reducing the users total bandwidth by about a third.

There are more details on +John Nack's post on the official Google blog:
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