That’s what head-canons are for.

@Daneasaur
 
Okay so RD might be the youngest. in “pinkie pride” she turn 21 because it’s in season 4 that she get her acceptance letter from wonderbolt academy so in the first episode of the serie she is 19, turn 20 at some point before the end of season 3. And Fluttershy might be the eldest nearing now her 30 as in the flash back she started her awkward growing years so around 15
 
Also the CHS world’s time go slower, (3 time slower according to what is seen in the comic) otherwise Sunset Shimmer would be in her mid 30’s, her human counterpart probably is.

@Background Pony #305E  
Spike’s growth rate is unknown and gauging his maturity is extremely difficult as he seems to occasionally be sage like or completely inept at any given moment.
 
However, the pony selves of the characters are OLDER than their EG counterparts, despite sharing certain traits. One of the panels mentioned that Twilight was around 20 when she moved to Ponyville and was somewhere in the range of 6-7 when she hatched Spike, making Spike about 6-7 years younger than her, putting him around 13-14 at the start of the series, and at LEAST around 14-15 during EG.
 
the EG crew is around 17-18 if I’m not mistaken, several years younger.
 
So spike being around 15, jumped into the body of a human, would be around a 15 year old male structure compared to a 17-18 year old female structure.
 
Around mid teens, some people don’t do a whole lot more vertical growing.
 
Add to it that Rarity would be of a more delicate frame anyway and it’s quite possible that, had spike become human, he could have been around the same size, if not taller.

I’d wanna be the dog. He gets a good view!

Even if Spike was a human, why would he be taller than Rarity?

@Max16032 Or maybe they will, and this was a subtle sign forwards!  
We’ll just have to wait and see if Rarity gets a sudden obsession with peanut butter.

The fact they made him a dog in the movie is a sign they’ll never made this pair canon. Sparity is just a gag.

Bad enough they made him a pin cushion but now this!? Spike happens to be my favorite character of the whole show

Notice how he’s looking up her skirt

Did the gala episode teach you nothing?

I didn’t even Expectied what you’ve “expectative”d

Good.

I know that feeling…
 

@Fortune Bad Pickup Line Spike: Now 20% more awkward.

The reason? There can be only one “handsome nice guy” in the movie, and you know who got that title……

“Think of it this way Rarity. At least now we’re both mammals…”  
“Gah…”

You know exactly where that mutt is looking…