This may or may not be a well known trait of this wild pet, but I was completely in the dark. Unless I'm using the same pet as the one that I'm facing, I often don't notice any size differences. Maybe I'm just not that observant. :P
After I found out about this neat little detail, I went out and captured the two sizes that I was missing. To reduce the guess work, I added my existing gryphon pets to my team so I could compare it with any of the wild gryphons I came across.
A great tip that Peanutty mentioned: if you find the Wildhammer Gryphon Hatchling as the secondary pet in battle, but it's not the right size that you were looking for, simply forfeit and reengage the battle. Much like different wild pet colors, disengaging and reengaging the match changes the secondary pets' attributes.
Once the new gryphons were secured, they were immediately upgraded to rare and leveled to 25. I normally don't name my wild pets (there are just too many!) but these guys were special. Since I don't know any trio of famous/noteworthy gryphons, I named them after Scrooge McDuck's nephews from DuckTales (Huey, Dewey, and Louie). If Blizzard adds three duck pets in the future, I may have to find different names for them. XD
The best way to compare and view their size differences is by putting them all on one team and engaging in a pet battle.
During the battle you can easily identify the largest of the three, while the middle-sized and smallest one are harder to tell apart.
The BEST way to see the size variation is at the end of the battle, when all three line up in a row.
That moment is fleeting though and I had a hell of a time trying to get a screenshot of their lineup lol.
It's been a while since I had something to collect, so this was a fun mini-adventure. If there are any other wild pets that come in different sizes (and maintain the varying sizes after capture), let me know! There's no question about it, they MUST be added to my pet army. :3