@Background Pony Number 17  
You ain’t kidding. Binged on his stuff back in 11th grade, and the mindscrew never quite wore off. I dimly recall a storyline where Xanth’s ambient magick tap was shut down somehow, and things got more (conventionally) natural & inconvenient (’taurs with upper-torso back problems, et.al.) for its sundry exotic denizens ’till the heroes restored status quo.

In Piers Anthony’s Xanth stories, he had a macguffin called an “Accomodation Spell”. Basically, it was a spell you could cast on two creatures of different species. The spell would take their physical characteristics and temporarily “average” them, producing two hybrids who would be physically similar for as long as the spell lasted.
 
The purpose behind this spell, needless to say, requires no explanation. The land of Xanth was full of “hybrid” animals, including demi-humans such as centaurs. Piers Anthony was a very peculiar author.

@Kolbjorn  
Yeah, half the fun of a readable Spikefic involves research/training/questing/loopholes to make that dragony mojo work for instead of on him.

@gorkspewdium  
Well, I wrote a fanfic where Twilight’s reasoning was: “If small Spike can make his body big, maybe big Spike can make his body small” It worked :P

@FloydPinkerton  
@Darth Sonic  
One’s a telekinetic, the other’s got a hyper-prehensile tongue for days. Fitting Traditional Tab A into Traditional Plug B might not be a pressing priority. Plus, as noted, in-law who reversibly shrinks & morphs her friends all the time.

@FloydPinkerton  
Well, she can just use her whole body to compensate.

@Darth Sonic
 
Rarity: Would you happen to know a size-changing spell that’s safe to use on, say, a dragon or a pony?
 
Twilight: Not off the top of my head, but I could certainly look into it. Why do you ask?
 
Rarity: Well…First off, I’ll say that Spike and I are very happy together. I don’t mean to suggest otherwise. But there’s a certain…how do I put this delicately…mutual feeling of physical unfulfillment in our marriage.
 
Twilight: I’m afraid I don’t follow.
 
Rarity: I mean to say there are certain…activities in which a husband and wife commonly partake that have never been possible for the two of us due to the size disparity.
 
Twilight: I still don’t understand. What activities?
 
Rarity: Perhaps I’m phrasing this too delicately. I’m hoping that some magic spell can make Spike and I the same size, even if only for one evening. That way we can do what we should have done the evening of our wedding.
 
Twilight: Oh! I get it now! You want to have a romantic dinner together, but Spike’s too big to sit at a table.
 
Rarity: (facehoof)

@Darth Sonic  
There is a name for that kind of situation: Hot Skitty on Wailord action

Sex must be tough.