There’s a limit to how big RSS feeds can be. It’s not a standard size but rather a limitation of feed readers. I believe Hugo tries to follow the size limit as imposed by Google. I’m not sure Hugo goes about it the most intelligent way, but it seems to do the job.
It actually would be useful if there was a way to over-ride this. It destroys our podcast in iTunes as our back catalog is now limited. I know that there are the usual limitations of feed size, but podcasts are special use case.