Low ceiling in my (originally set up to be) "drum room", would be the culprit for me here then!

That's why I end up with a wide image, because I have the mics a little closer to the source than I'd like. Which is fine, becuase if I want wide I can have it, and if I want narrow, a pan them in narrow.
Rami, The reason for the wider image being due to the mics being too close (not too close together, but too close to the source), is because mic Left is picking up more sounds more prominently that are directly in front (below) of the mic, and rejecting more of the stuff over to the right. And vice versa for the right mic. Each overhead is more isolated.
PICTURE THIS: You have your overheads in a spaced pair (or xy or or any stereo technique for drum oh's). Now raise them up 100 feet above the drum kit. The stereo image will likely be very unclear. Since the drums are so far away from the mics, the floor tom will likely be just as loud in both mics, and same goes for anything on the kit. Therefor a narrower image. Less distinction between what came from what side of the kit.