Normally, the % mark says that the file might be Base64 encoded after encrypting, and also might get some checksum over each block to prevent corruption.
Base64 encodes characters of 8 bits into characters of 6 bits, which means the file in question gets about 30 % larger due to more charachters required to render the whole file. Add a per-block checksum and you are up to 35 %.
Normally, the encryption itself adds some overhead. Normally, the overhead is header+footer, eventual encrypted key, parameters, salts, checksum, and also one block size minus 1, because if the encrypted data is not evenly dividable with the block size, you would have to pad with up to block size - 1.
But all those data in the previous sentence would add a static amount of data to every file, regardless of its size, even if its 1 or 100 GB large.
The data enlargement expressed in % says its a reencoding process like base64 or something similiar.