why do files downloaded with http and https have the same size?
[After your edit I can confirm that the browser does the whole encryption in the background - you only get the "normal" HTTP connection to see and the downloaded files appear as big as the unencrypted files - even if the encrypted files are only marginally bigger. You don't ever see the encrypted version without some addition work. You can use a secure connection like a normal one.]

Here a explanation why the exchanged data is slightly bigger than with unsecured connections: Ciphertext is mostly only slightly larger than the plaintext, not a significant part if the plaintext is already pretty large. At first (with for example TLS) we create a session which exchanges some data to authenticates the connection and generate a secure session key. This takes not much bytes, maybe a kilobyte. After that only a IV (ca. 16 bytes) and the padding (ca. 1 to 16 bytes) needs to be exchanged - only below ca. 4 percent size increase with 1 kilobyte exchanged plaintext. With bigger file sizes this will be even lower.