Alright, I've finally gotten some screen shots going to help illustrate the differences in pet sizes. There are some further ones I'd like to take, but it's honestly quite tiring to level and relevel pets simply for screen shots. I've done about all of that I can take at the moment.
Using two of my characters as a measuring tape, I took screens of them with both the normal level 3 Moa Bird and the evolution in question. The hard part was to get a screen where the Moa is at a neutral position in regards to the character. It can not be closer to or further from the camera than the character; nor can it be standing on a higher or lower plane from the character. Considering the amount these buggers move around, that's a lot more time consuming than one would think.
Necro with Lvl 3 Moa Bird
Necro with Lvl 15 Dire Strider
Warrior with Lvl 3 Moa Bird
Warrior with Lvl 15 Hearty Strider
Then, overlaying the two images in photoshop, I resized one of them and moved it about until the two images of my character matched up precisely. This creates a ghosted image of the two variations of Moa Bird on top of one another for very easy size comparison.
Necro with ghosted Dire
Warrior with ghosted Hearty
Lastly, I took the these two ghosted images and once again overlaid them in photoshop. This time I adjusted them so that the level 3 Moas both matched up. Then I drew a red line over the Hearty Moa indicating its size, duplicated it, and dragged it over top of the Dire as well. As you can see, the two are clearly the same size.
Final ghosted image with both Hearty and Dire Striders
This should settle the question as to whether or not the Hearty pet is larger than the Dire pet. The two are the same size.
As to the confirming that the Playful pet is indeed larger than the normal pet, I have a couple more screen shots. The Warthog, which is best for noticing size variations, can be seen here in poison. Careful to use the same spot so as to avoid varying depths, you can see the size change from normal to Playful, and then again from Playful to Hearty.
The
normal Warthog is almost completely submerged with only the tip of its tail and the very fringe of its mane showing.
The
Playful/Aggressive Warthog shows the tips of its ears and considerably more mane than the normal one.
The
Hearty/Dire Warthog shows all of its ears and some of its light-colored back as well.
Edit: confirmed my suspicion that Aggressive and Playful pets would be of equal sizes, and thus updated my post accordingly