I find it very curious whenever I see, or read about the Ponies being roughly the size of medium sized Dogs when compared to humans in fan-art and fan-fiction.
I always imagine them being ... oh ... I don't know ... PONY sized.
You know ... smaller than Horses, but bigger than most humans, and large enough for young children to ride on their backs. Standing on all fours, they would be shorter than the average human, but still not itsy-bitsy by any means, but significantly sized animals roughly around the size of ... well ... a Pony.
If that [the size you depict Luna here] was how big they are, that would make Spike pretty darn small, and Winona, Angel and Opal-Essence would be downright TINY. Especially considering how much taller Luna and Celestia are in comparison to the other Ponies.
... just sayin' ...
H-Luna: "Ugh ... WHY are we doing this again?? Do you have any idea how bad of a Moon-Burn I got after laying up there for a Thousand Years? I just finally got my skin to stop flaking."
P-Luna " *Sips Casually* ... Wouldn't know ... Ponies don't get Moonburns OR Sunburns ..."