The default piecesize in mktorrent is small - 256 KB i believe. Try this syntax:
the -a is announce URL; is equivalent to --announce=
-p is privacy flag
-l 21 is piecesize of 2 MB, will make the .torrent significantly smaller
-o is to name the .torrent to whatever u specify
NameOfRelease/ is the directory of the torrent you are trying to make. Note that you have to be in the parent directory of the NameOfRelease/ folder for this part of the syntax to work. If you're not, use the full path to the torrent data.