minimal overhead of ChaCha20 + ECC What we want?

Encrypt plaintext with ChaCha20 + ECC on curve 25519 (hybrid cryptography) the result would be a data stracture X.

Input: public key on Curve 25519

No authentication required (no AEAD)

What is the minimal storage ovehead of producing a data stracture X as compared to the plaintext. 
What are the required steps, and what are the elements of X.

The resulting data structure X should be decryptable by requiring only the corresponding private key.

If we were to cut on storage ovehead even further, at the cost of security (change curve Curve 25519 to something else? idea?)
32 Bytes (+16 for the tag)
Let data be your data, let || denote concatenation and let pkD be the &quot;data's public key&quot; (that is a Curve25519 public key which has exactly 32 bytes) and tag be the Poly-1305 authentication tag (16 bytes long).
Because pkD and tag have a fixed size, you don't need to encode their lengths and you can literally store this as pkD||data||tag. For decoding, you just grab the first 32 bytes as the data's public key and the last 16 bytes as the authentication tag.
Encryption would go like this:

Run a standard Diffie-Hellman key agreement against the recipient's public key. The public key you would normally send over the wire is pkD and will be stored for decryption.
Feed the shared secret into a key-based key derivation function to get an IV, a key for the symmetric cipher and a key for the MAC.
Encrypt the data using the symmetric keying material and append the authentication tag.
Delete all keys and the data.

If it is not of concern that an attacker can flip bits in targeted attacks with bit-level accuracy (that is, he can modify any known plaintext-ciphertext pairs to say whatever he wants), then you can omit the authentication algorithm and the MAC tag.