“Me and shawty go together like chemicals”-Lil Wayne
Anasteemaphilia is the attraction by a person to people either much shorter than them or much taller than them. Depending on the extent of the fetish this means that some people will only enjoy sexual activity with someone who is significantly shorter or taller than them. Anasteemaphilia can also mean being attracted to dwarfs but not necessarily. In a lighter form, a really tall man may only like women who are more than a foot shorter than him.
Not at the same level
Anasteemaphiliacs are literally having sex at different levels. Different levels of altitude that is. When you see someone who is more than a foot taller than their partner it is a common thing to wonder how they get the logistics of sex to work. Honestly, the logistics are not necessarily that different. Yes, missionary position might make the smaller person feel a bit smothered but if the larger person is up on their elbows then it should still be fine.
From the psychological perspective there are theories that the reason some short people are attracted to really tall people is because the difference in size helps them to feel safe and secure. In reverse, the tall person gets to feel like they are taking care of and protecting the smaller person. Some of it can come from getting really tall really fast when you are younger, so all of your peers are significantly shorter than you. Since this is when you are forming your personal sexual language, it leaves some people with an attraction for height difference.