We have noticed that sitemaps can now be 50MB up from the previous 10MB. The change was made to manage for longer URLs which are becoming more common with database driven sites. We use descriptive permalinks which make it easier for people to read when they wish to save a URL.
The sitemap is still limited to 50,000 URLs which is fine for the majority of sites out there. Some larger sites may need to break up the map into parts to manage with a big database.
We use a plugin with WordPress that dynamically creates a new sitemap on demand. This means any search engine that reads it will have the latest version on hand.