updating on USB live installation

[mma8x]

i'm using kubuntu karmic on a 2 gig usb drive. upon first installation, i have about 1.1g free. however, when i try to upgrade the packages, all of the free space is taken up. if i try to remove all uneeded packages, i can manage about 100 mb of free space. so 1 gig is sucked up by the upgrades, which doesn't seem right.
a little more info:

ubuntu@ubuntu:/$ sudo du -h --max-depth=2|sort|grep M
du: cannot access `./proc/18420/task/18420/fd/3': No such file or directory
du: cannot access `./proc/18420/task/18420/fdinfo/3': No such file or directory
du: cannot access `./proc/18420/fd/3': No such file or directory
du: cannot access `./proc/18420/fdinfo/3': No such file or directory
112M ./usr/bin
11M ./rofs/sbin
124M ./lib
124M ./rofs/lib
13M ./boot
13M ./lib/firmware
1.3M ./lib/udev
14M ./cdrom/pool
155M ./var
1.8M ./var/log
18M ./var/tmp
2.0K ./etc/NetworkManager
28M ./usr/sbin
2.9M ./rofs/boot
31M ./home
31M ./home/ubuntu
38M ./var/cache
4.5M ./etc/brltty
5.6M ./bin
5.8M ./rofs/bin
599M ./usr/share
688M ./cdrom/casper
7.0M ./rofs/etc
735M ./usr/lib
7.8M ./etc
78M ./rofs/var
95M ./lib/modules
9.5M ./sbin
98M ./var/lib


deborphan doesn't free anything, cache files are clean. any other ideas?