I’ve logged this on Show xx of 250MB on AnkiWeb · Issue #3017 · ankitects/anki · GitHub
After deleting content, you need to 1) use check database and then 2) force a full sync in order for the space to be freed from AnkiWeb. The limit is 250MB, and if you don’t have a millions of previous reviews, chances are the majority of the space is taken up by large text content in the fields (or verbose HTML formatting).