dark-souls
When you first find Sif, he is weilding Artorias' greatsword, and the thing is as long as a bus! However, when you find him later in the DLC, he is only around twice the size of the Chosen Undead, and his sword is much smaller (wink).
Is there anything in the lore about how the sword grew/what caused it?
Found a solution on the Steam game hub..(scroll to bottom) http://steamcommunity.com/app/211420/discussions/0/882959061668056366?insideModal=1
Also, a more detailed source..(tip #5) http://segmentnext.com/2012/08/24/dark-souls-pc-crashes-low-fps-fix-controls-errors-freezes-and-gfwl-fixes/
The trick is to disable any of your devices listed as an "HID-compliant game controller". No need to uninstall. I created a system restore point JIC.
Control Panel > System > Device Manager > Human Interface Devices >> [Disable all of your devices listed as an "HID-compliant game controller"]
Remember to restore them when you're done. Quite a pain :| All things considered, its working now and that's fine with me until its patched.
If I am thinking of the same giant you are referring to, this is how to get to him...
Starting from the beginning of the fortress, after you get past the gate entrance with the two lizards, you enter the start of the guillotine area, drop down to the left and enter the little entrance way shown in the picture.
Climb the ladder in there to the top and there should be a secret entrance behind an arch. Get through that, climb the ladder behind it and it takes you straight to him.
Here are some pictures to point things out, the pictures follow the order of events I described. So it should be self explanatory to get to the giant, hope it helps out.
Best Answer
A general, rather than Dark Souls-specific answer: It is a common trope that gods can take whatever shape they want, but they are still slaves to narrativium. So they usually end up looking like big humanoids when they want to intimidate. From there it's not much of a stretch to imagine that any weapon a god wields will change to fit them.
A lore-based theory is that they are actually different swords, and the one that Sif is wielding is actually part of Sif.