Okay, I haven't tried running Borderlands since the update myself, and I did NOT get the DLC. I checked only for changes that happened automatically with the Steam update. Here is what I found (not a complete list, there's also some package stuff in CookedPC and maybe more).
First, note the new localization entries. These contain stuff like mission text for the DLC.
Next, note some small packages regarding the DLC. Maybe this handles how missions are integrated. I'm not sure.
Next, note a DLC installer executable. I didn't run it.
Lastly, and most concerning to me, this patch automatically updates your Borderlands executable to 8MB or so larger, if I'm not mistaken. And take a look at the comparison. No SecuROM mention in the original file, and hey, a bunch of SecuROM mentions in the top file! SecuROM v7? Silently in an automatic patch? I'm not sure, but it doesn't look out of the realm of possibility!
So in conclusion, yes, the actual DLC resources for levels and textures and stuff is probably large and probably only downloaded when you buy the DLC. But yikes, this automatic silent patch seems a little concerning to me. :/ (seems it might cause sound/graphics glitches as well as erasing your .ini file tweaks)
*edit*
And to update again, if you check the Steam page for the DLC, the DLC ITSELF has a SecuROM 5-installation limit...