curl normally displays a progress meter during operations, indicating the amount of transferred data, transfer speeds and estimated time left, etc. The progress meter displays the transfer rate in bytes per second. The suffixes (k, M, G, T, P) are 1024 based. For example 1k is 1024 bytes. 1M is 1048576 bytes.
curl displays this data to the terminal by default, so if you invoke curl to do an operation and it is about to write data to the terminal, it disables the progress meter as otherwise it would mess up the output mixing progress meter and response data.
If you want a progress meter for HTTP POST or PUT requests, you need to redirect the response output to a file, using shell redirect (>), --output or similar.
This does not apply to FTP upload as that operation does not spit out any response data to the terminal.
If you prefer a progress “bar” instead of the regular meter, --progress-bar is your friend. You can also disable the progress meter completely with the --silent option.