I have a website with millions of links. I want to create an efficient and effective sitemap. There are some type of sitemap, xml, xml.gz, txt and txt.gz. I think txt.gz is the smallest size but is it better then xml.gz for search engine?
URL list text files are smallest since there is less fluff. You can check this sitemaps comparison article. It has explanations and code examples of all sitemap file kinds.