Google and Bing both has announced a change to the sitemap file size limit, increasing it from 10MB to 50MB. That means the uncompressed version can now be more than 10mb but must be under 50MB.

It is important to note that the 50,000 URL limit per Sitemaps file has not been changed. That is still limited to 50,000 URLs in a single Sitemap file. but the file size has increased drastically because there are some other aspects of sitemaps that are bringing the sizes of these files up.

Source: http://selnd.com/2gwLg9k