Error indexing posts: Limit of total fields in index has been exceeded

When I run:

```
$ wp searchpress index --flush --put-mapping
```

I eventually see this error:

```
Warning: Error indexing post 5021340; HTTP response code: 400; Data: {"index":{"_index":"bgr.test","_type":"post","_id":"5021340","status":400,"error":{"type":"illegal_argument_exception","reason":"Limit of total fields [1000] in index [bgr.test] has been exceeded"}}}
```

The stats end up as:

```
Completed page 1/1 (29.36s / 101.48M current / 157.01M max), 7s remaining
Success: Index Complete!
1481 posts processed
1081 posts indexed
400 errors/warnings
Replacing the default search with SearchPress...
Success: Successfully activated SearchPress!
```

Any ideas why there's a cap?