Files are cut into chunks of up to 5 MiB, padded to 4 KiB, and each chunk is encrypted with its own random key, so copies look different and chunks are unlinkable without the read capability.

Chunking plus random keys is what hides sizes and structure, not just contents.

A file is cut into pieces of up to five mebibytes.

Each piece is padded to a multiple of four kibibytes. So the server never learns the exact size.

Each piece is encrypted with its own random key.

The key is random, not derived from the contents. Two copies of the same file look nothing alike on the server.

And the pieces of one file are not linkable. Without the file's key, nobody can tell which blobs belong together.

Check yourself. Why is a chunk's key random rather than derived from the chunk, and what does the server learn about a file's size?