1) Upload package file "mypkg.mgnl" to Magnolia

2) After completion: ls -l <magnolia dir>/tmp
rw-rr- 1 x x 800980 May 11 17:38 mypkg.mgnl

3) Right-click mypkg.mgnl in Packager and choose Install

4) During installation: ls -l /tmp
rw-rr- 1 x x 800980 May 11 17:38 mgnl_pack3891562675922581253.mgnl
rw-rr- 1 x x 904803 May 11 17:38 mgnl_pack4609735942257620967.mgnl

5) After completion: ls -l /tmp
rw-rr- 1 x x 904803 May 11 17:38 mgnl_pack4609735942257620967.mgnl

So it seems the Packager is first making a copy of the source package file, and then process it into another file, cleaning up only the first one. The left-over files accumulate and fill up the tmp partition.