JohnnyBGoode 
"[@Zaknel":](/1707747#comment_7061060
)
Im no sociologist, but I've a couple theories:
1) People tend to associate height with maturity, and (personality wise) Rainbow Dash is one of the most immature members of the mane six (second only to pinkie pie)
2) An attempt at applying real world logic to the characters physical abilities. To me, applejack would be the "strong" one because the show depicts her doing heavy lifting via farmwork. Rainbow, on the other hand, has always been portrayed as the speedy athlete archetype (same reason she is typically drawn flat chested). This doesn't really explain the height issue, but it does explain why she usually isn't drawn swole (at least not at the same rate as Applejack)
3) Napoleon complex humor. Rainbow is (again, personality wise) the most aggressive one of the mane six. And having the toughest talker be the runt of the group is a funny mental image. Also, some men/lesbians find that type of girl to be cute.
4) As a caveat to #3, some insecure dudes might find an aggressive girl who is taller than them to be threatening to their masculinity, so they draw her short in an attempt at making her a tomboy but still distinctly feminine.
Im no sociologist, but I've a couple theories:
1) People tend to associate height with maturity, and (personality wise) Rainbow Dash is one of the most immature members of the mane six (second only to pinkie pie)
2) An attempt at applying real world logic to the characters physical abilities. To me, applejack would be the "strong" one because the show depicts her doing heavy lifting via farmwork. Rainbow, on the other hand, has always been portrayed as the speedy athlete archetype (same reason she is typically drawn flat chested). This doesn't really explain the height issue, but it does explain why she usually isn't drawn swole (at least not at the same rate as Applejack)
3) Napoleon complex humor. Rainbow is (again, personality wise) the most aggressive one of the mane six. And having the toughest talker be the runt of the group is a funny mental image. Also, some men/lesbians find that type of girl to be cute.
4) As a caveat to #3, some insecure dudes might find an aggressive girl who is taller than them to be threatening to their masculinity, so they draw her short in an attempt at making her a tomboy but still distinctly feminine.