Each MSDOS formatted floppy allocates 512 byte sectors, 2 sectors per cluster, and 112 or 224 entries for filenames in the root directory. By reducing this to 512 byte sectors, 1 sector per cluster, and 96 root directory entries, you can gain a few more kilobytes on the disk. This is done with some options to mkfs: mkfs.vfat -S 512 -s 1 -r 96 /tmp/disk.ima

The S option specifies bytes per sector, the s option sectors per cluster, and the r option root entries.