At least on my server, after I made the change to the php.ini file, I had to then copy that file into all of my Round Cube Folders (at least the main ones). Something about without there being a php.ini file in that specific folder, that any php function run out of that folder defaulted to the server settings.
FYI, I only needed to change the php.ini file (upload_max_filesize & post_max_size), not the .htaccess file.