I use Hg for source code, and was previously committing my artwork into the same repository, but I've come to the conclusion that this isn't a good idea, because the repository is growing too large.

I use SVN for source and art. Yes, it get bigger over time(currently ~8-10G), but I don't mind as long as it doesn't surpase the capacitiy of a blu-ray. Once it hits a critical mass I will most likely archive the art-repository and create a fresh new one from the last snapshot, but I never reached it yet

There are commercial tools (perforce), but does a small team (1-2 devs) really need such a tool ?

I'm using Git for code and SVN for art at the moment, which is a bit painful... The last company I worked for did the same. The one before that used Perforce.

I haven't tried them yet, but git-annex, git-bigfiles, git-media make it possible to manage your massive art files with git too.

There are commercial tools (perforce), but does a small team (1-2 devs) really need such a tool ?

Perforce is free for small teams (<20), only large teams have to pay for it

What's special about perforce besides basic versioning and bindiffs? I'm not familiar with the product, but I can't imagine anything else really being needed, and those two things alone are pretty easy.

It's pretty complex. I've only worked with it as a grunt, never the team leader or in charge of the project infrastructure, so I've only been a simple user... but the interface for it was huge. We had a single engine repo that was "linked" (like a shortcut/hard-link) into dozens of different game repos, and managing all of those branches and remerging the divergent versions of the engine seemed to be very simple.
It's also both centralized (like SVN) or distributed (like Git) or set up as a hybrid of both, and there's a tool to let it integrate with git so that your coders can use git but the server is actually a perforce database.

I started off only putting source in Git, out of fear of bloating the repository, though this had the downside that people couldn't run the game from source just from the repository, they'd have to download the latest released archive of the game - which is more hassle for them, but also a problem if changes have been made to support changes to the game code (obvious example being new graphics being added).

I didn't want the hassle of having to upload art separately; I looked into the various Git add-ons, but preferred something that worked with standard Git. So now I just stick the media that's distributed with the game into Git, and it seems to work fine. I sometimes leave large files like music out of it, as these are optional. Media that isn't distributed with the game ("source" files for art, concept art, and so on) can also be left out.

I am using git for all source, source code and assets. even though the art content change is not detailed by git.

I wonder if it would be too much hassle to use dropbox for the assets. I mean, if you don't need "proper" versioning for your assets.