Amazon does aggressively monitor and shut down accounts that store pirated content, but I've yet to hear any reports of Amazon cracking down on the amount of storage used.

There are plenty of people who are storing absurd amounts of data [1]. If Amazon were cracking down on this, you'd hear it in /r/DataHoarder first.

The ToS [2] don't mention any size limitations. In fact, it says you can store pretty much any file as long as the data doesn't violate any laws, including copyright.

The only really worrying part is this:

    3.2 Usage Restrictions and Limits. The Services are offered
    in the United States. We may restrict access from
    other locations. There may be limits on the types of
    content you can store and share using the Services,
    such as file types we do not support, and on the 
    number or type of devices you can use to access the
    Services. We may impose other restrictions on use
    of the Services.

[1] E.g. https://www.reddit.com/r/DataHoarder/comments/54bci8/anyone_..., https://www.reddit.com/r/DataHoarder/comments/3zhowv/amazon_...

[2] https://www.amazon.com/gp/help/customer/display.html/?nodeId...

I think Amazon drive is Amazons way to utilize unused storage and old storage capacity from S3.