If your aircraft had been turning away from the other aircraft, even if only slightly, the position of that other aircraft relative to the wingtip of your own would have made the apparent difference in height look much less. I have been in a holding stack 1000' below a bigger aircraft whose turns were a little wider than ours and as we both turned it really did look like we were flying wingtip to wingtip even though I knew we had 1000' separation.
Incidentally, 100' is roughly the wingspan of an A320.