I'm guessing you are seeing that in device manager. If so that is percentage for used bootflash memory, which isn't memory in the same sense as memory in a PC. It's just flash based storage that Cisco decided to call memory.
Generally the kick-start and system images are stored there. You may also have old versions of those files from previous code levels. Run dir bootflash: from the CLI and you will see a breakdown of that space. Example from one of my switches:
SAN-F-PROD-D10-9509# dir bootflash:
49152 Mar 24 17:29:24 2009 lost+found/
16493056 Mar 24 17:20:59 2009 m9500-sf2ek9-kickstart-mz.3.3.2.bin
16498176 May 27 08:22:23 2010 m9500-sf2ek9-kickstart-mz.3.3.4a.bin
79834400 Mar 24 17:19:14 2009 m9500-sf2ek9-mz.3.3.2.bin
79880630 May 27 08:19:08 2010 m9500-sf2ek9-mz.3.3.4a.bin
7595 Jul 16 07:27:03 2010 m9500-sup2dimm-gplug-mz.3.4.1.bin
Usage for bootflash://sup-local
255410176 bytes used
642678784 bytes free
898088960 bytes total
I would say it's user preference on how much extra you store in the bootflash, you at least need the current system and kickstart files stored. In my example we happen to have our current version of sys and kick start files and the previous version we were at. Basically this is just left over from an upgrade, just in case there was any reason to downgrade the switch quickly post-upgrade.
