Microsoft has recently made a significant change to how storage is allocated to site collections within SharePoint Online and also increased the number of external users from 10,000 to unlimited. Both changes are for Office 365 subscriptions only and do not apply to SharePoint Server

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To summarise, SharePoint site collections no longer have to be allocated a fixed amount of storage when they are created. Instead, they can be set to ‘Auto’ and will automatically grow within the total storage allowance for the tenancy.

This is a significant improvement for deployments that involve multiple smaller site collections.

To utilise this feature, existing site collections will need to be updated in the settings to switch from ‘Manual’ (fixed storage amount reserved for this site collection) to ‘Auto’ (automatically grow within the total storage allocation for the tenancy)

For new customers, Microsoft is configuring the service to be ‘Auto’ by default for all new site collections. Existing customers will need to amend this setting themselves if they want to switch from ‘Manual’

Full details are provided in the article, including instructions on how to make the required changes

SharePoint Online simplifies storage management (Microsoft Office Blog, August 2014)

And on a separate note, Microsoft has also updated guidelines for external users – the number is now unlimited for SharePoint Online:

View the updated Software boundaries and limits article on TechNet.