Cassandra writing (too?) large partitions
By default, Cassandra uses SizeTieredCompactionStrategy that compacts several files (4 by default) of similar size into bigger file. These files contain multiple partitions, so big file size isn't a necessary sign of wide partitions.

When you're using Cassandra (especially with SizeTieredCompactionStrategy) you need to have ~50% of disk space free so Cassandra will able to write data during compaction - after compaction happens, the old files will be removed.

You may also consider the use of other compaction strategies, such as, LeveledCompactionStrategy, but it's more suited for read-heavy workloads (like 90% of all operations are reads).

You can read more about compaction strategies in documentation, and about size tiered compaction in this blog post.