The data sources referenced by layers in your map are not registered on the server. When a data source is not registered on the server the data referenced by a layer will be copied to the server. Only data within the data frame's map extent will be copied to the server. When the data frame uses the full extent, all the data within this extent will be copied. In many cases, this may be more data than you want, or need, to be copied.
You may have datasets within your map that cover a larger geographic area than the area you want your map service to display. If you are copying data onto the server you may be copying much more data than needed. This may have unintended effects on performance. In most cases, you will want to use a custom full extent to define just the area you are interested in. This way, you will only copy a subset of the data to the server.