I'm playing in a all ocean world in 1.11.2 and since the islands are so far apart, I decided to connect them using a minecart in the nether. Just finished a track connecting a pair 2800 blocks apart in overworld but only 350 in nether. Simple one-way track but does have 75 block elevation change. To prevent Ghasts from blowing it away, i's all underground. Each end terminates in a 13x15 room with 4 block high ceiling connecting to outside via a 3x3 entrance tunnel.
Just went down it and was really surprised to find 3 mid-sized MagmaCubes and a Ghast waiting for me in terminus room. Now I suppose the Cubes could have bounced in, but how the heck did the Ghast get in there. It was so big it couldn't even turn around to blast me, and sure too big to go through a 3x3 tunnel. Did the darn thing spawn underground?
If the terminal rooms arent half slabbed or don't have a floor of transparent blocks i think its possible to get Magma cubes spawn. Ghasts also if the space is big enough as you found out. My first Nether minecart station had a largish space in the roof above the redstone and one day i found a Ghast wedged in the corner. It was funny til he fired and damaged some of the redstone, so i made short work of him. When i rebuilt the station years later using a more compact redstone system, i had an unused space left about 18 x 5 x 5 blocks and I had a few ghasts in there too til i half slabbed it.