After learning about this I set out to capture a second bullfrog. My original one is the large version, but it didn't take me very long before finding a small one. I quickly nabbed it and then placed both the original and the new bullfrog on the same team to get a better size comparison (like I did with my gryphon hatchlings).
Only then did I notice something odd occurring. The first battle with my froggy team went as planned. Big bullfrog went first, small bullfrog was second. Great, but oh drat, the screenshot wasn't the best. So I tried again. The second battle had me scratching my head since both frogs looked exactly the same size. So I tried again. And again. And again.
To confirm my suspicions I ended up simply summoning out my original "large" bullfrog and placed it next to random frogs.
The images above show only one bullfrog next to the wild one (the green one is mine, red one is wild). Yep, my original is a size-changer.
So far I seem to be the only one with this issue. One possibility, as Peanutty suggests, is that my frog was a secondary pet in battle and captured before the fix was implemented so that pets retain their captured state (color and size).
The dilemma now is do I keep the second Yellow-Bellied Bullfrog that hasn't had any issues with maintaining its small stature, or do I release it since I have kind of have both sizes in my original frog? I'm leaning towards releasing the new one. If they ever fix my size-changer, it's not as though it will be too difficult finding another bullfrog, right? I can easily go out and capture the size that I don't have.
This incident makes me even more curious about the hundreds of other wild pets out there. Are there any other pets in my collection that are size-changers too? What sizes am I missing? If only there was an efficient way to check them all. I guess I'll have to continue relying on the awesome pet collecting community to keep me informed. :)