I have an EeePC 701 4G running Ubuntu Netbook (10.04). I installed the system on its 4GB SSD for speed reasons - fast SD cards are expensive. I keep all user data on a fairly slow SD card.
Problem is, after a few updates, I have less and less free space on the SSD.
Since updating replaces old packages with newer ones, shouldn't the size remain more or less the same? I'm almost out of space and will probably have to stop updating the system soon... and yes, I do perform apt-get clean after every update. It helps, but the size is still steadily creeping upwards.

Try:

Unfortunately, I don't know of an operating system that doesn't creep upward in size over time. I have observed the same behavior with Windows and Mac systems, too. 4gb is really too small for Ubuntu in my opinion and you might be better served with a lighter-weight distro.

It might be a good idea to try uninstalling original packages. You can also do the following

in /tmp: rm -rf *.tmp *.part ssh* (not if you are in an ssh session) and any old copies of other directories multiplying away in /tmp.
dhcpcd makes files with weird names and they can go - careful with the one with today's date!
Do not do rm -rf /tmp/* You have been warned.
/usr/share/doc & /usr/doc can be cleaned out of READMEs, INSTALLs, and 500 copies of the GPL, and everyone's docs in latex, html ps & pdf or foreign languages
Directories in $MANPATH can be cleared of languages you don't read
With the exception of your X driver, You can uninstall xf86-video-*
/var/log/whatever you don't need. If you don't need logs, you can go
> /var/log/messages (etc) which doesn't delete them, but zeroes them - they become 0 bytes.

You'll have a bit of space then. Don't make mistakes at this stuff. There's no way back.

Problem is, I've found Ubuntu Netbook to be the perfect match for the 701, as the Fisher-Price "My First Interface" look works great on the tiny 7" screen. It used to have Debian installed before that, and its standard desktop was painful on such a small screen. I also gave a go to Puppeee (software selection too limited) and Lucid Puppy, but while they're decent distros, and plenty fast as they run entirely from RAM, I can't stand the way in which they handle the desktop. Package maintenance also becomes really messy if you deviate from what's natively available in Lucid Puppy and use Ubuntu packages.

Were the computer still somewhat current I'd expand it with a larger SSD or fast SD card or... something. However, as it stands any upgrade would cost a significant fraction of the value of the whole computer, so it strikes me as economically unsound. If worse comes to worst I'll just resign to an unupgraded system.

Thanks. I'll do a full dd backup of the partition before I do that so I can rescue it if I screw up.

As you seem to have a real machine in the background,

I have done linuxfromscratchhttp://www.linuxfromscratch.org The base system is ~1 gig with X installed. I would recommend the CLFS variant, and compile on the bigger box for the smaller one. Then add what you want.

I also imagine if you simply uninstalled what you don't use, you could survive nicely. About 15 years back I had a stripped 32 bit system with X & kde in 250 megs on a laptop. No bloat; If I didn't know what it was, it was deleted. I didn't run out of disk.

Given your disk size, all compilers, debuggers, locale settings for klingon and the 4 corners of this world, everything in /usr/include (= header files - No compilers, remember), Howtos, vi or emacs, countless competing window managers, and every piece of software you don't use are lying there taking up space and not earning their keep. Handiest place to start in with your package manager. Libs come with one or two files in /usr/lib and a package telling programmers how to use them. Keep the lib, dump the docs.

This box has everything but the jacks installed and stuff I clearly will never use, and it's at 4.8 Gigs on /. I could probably halve that, and get it under 2 gigs by the sort of economy I have recommended to you. Go to it.