In '/var/tmp/kdecache-$USER/kpc' I have several 120.5 MB .data files, one for each plasma theme. What exactly do these files contain/what is their purpose?

I believe it stands for KDE Pixmap Cache. You can safely remove all of those when Plasma is not running, it will regenerate the ones it requires on startup.

A big bulk of it is Plasma theme caches, which you can safely delete. I wrote about it here.

Adding to that, these are sparse files, their apparent size is huge while the size they actually take on the HD is small, e.g. on my machine:

$ du /var/tmp/kdecache-user/kpc
14M /var/tmp/kdecache-user/kpc

$ du --apparent-size /var/tmp/kdecache-user/kpc
179M /var/tmp/kdecache-user/kpc

Dolphin/Konqueror will report the apparent size, 179M.