Big objects are objects that store and manage massive amounts of data within Salesforce without affecting performance.
If you wish to compare or deploy them, you need to ensure Custom object is included in the metadata filter of your comparison.
Custom object metadata is included in all the default (pre-defined) metadata filters.
Below we're showing an example of 'Big object' retrieved via Custom object metadata:
On the comparison results page, you can quickly narrow down your results to only show Custom object by filtering the Metadata type.
You should now see your Big object, which can be included in your deployment.
Why can't I find my 'Big Object'?
If you're having trouble finding your Big object, your object might be missing an index.
Without an index, the object isn't retrievable via the Metadata API.
Once an index has been added, you can refresh your comparison in order to deploy your object.
Read more about refreshing metadata in the comparison:
Refreshing metadata in 'Compare and deploy'
Note: In order to create an index, your object will additionally need to have at least one Custom Field.