The 4:3 ratio
Base64 encodes 3 bytes (24 bits) into 4 characters (each carries 6 bits). That means output length is roughly 4/3 of the input — about 33% larger.
Padding with = appears when input length is not a multiple of 3, adding up to two extra characters at the end.
Practical impact
A 3 MB file becomes roughly 4 MB as base64. For APIs and email this overhead is usually acceptable compared to the reliability of text-safe transport.