What version of Hugo are you using (hugo version)?

$ hugo version
hugo v0.97.3+extended linux/amd64 BuildDate=unknown

Does this issue reproduce with the latest release?

Yes,

If you create a sitemap on a site with over 50K urls , Google complains that the file is too big.

Your Sitemap contains too many URLs. Please create multiple Sitemaps with up to 50000 URLs each and submit all Sitemaps.

I looked at the docs and noticed that there is no way to override this. Technically not a bug, but this makes it difficult to submit sitemap to Google.