Not the OP but I definitely also had the same sensation when I was a child and young adult, especially when I had a fever. Although it was mostly a sensation that I was in a white (or amber) three-dimensional space with black objects floating around that alternated between being millions of miles away and just millimeters, and I could stop that by opening my eyes.
I think the fact that you even knew what chickens were when you were less than 18 months old is rather remarkable.
Hmm. Looks like we really only have anecdotes. The explanations listed on the wiki page all seemed like interesting hypothesis, but only with pretty flimsy experimental evidence backing them up. Personally, I’d lean towards the idea that it’s a matter of brain development. But there doesn’t seem to be much evidence here either – particular areas of the brain (the hippocampus and prefrontal cortex) are important for memory formation in adults, and these areas are still developing until the age of three or four. That’s an intriguing bit of evidence, but it’s entirely circumstantial, and you could also point to any number of other physical bits that are still underdeveloped during infancy.
While everyone else is tossing out their anecdotes, I might as well add mine to the pile. I have a very distinct first memory of consciousness, though I’m only in my 20s and it’s fading (it used to be very vivid). Still, I remember waking up, toddling downstairs and seeing my parents, and feeling that I had never seen any of this before, while it still seemed very familiar. That was probably from when I was 2.
I do have plenty of memories from shortly afterward, mostly dealing with my sister’s birth when I was two and a half. Some of which are confirmed by my parents, and others having to do with when they had a friend babysit while they were at the hospital.
I’ve had this same thing happen to me infrequently throughout my life! Its the absolute oddest sensation, one that i could never put to words, but your description is pretty spot on. Happened just a few weeks ago, in fact, and was going to ask about it here, but had no idea how to describe it. 
Me too. Wow, I never knew that it was (semi) common. i guess there’s different manifestations in different people but for me there was definitely the same sense of the infinitely big and the infinitely small at the same time. I think mine was weirder though!
For some reason I was eating a cube and it was obviously small enough to fit in my mouth, but then it would alternate to as big as the universe. It got really nasty when it was already in my mouth and switched to insanely massive. My reaction to it was … hard to describe. The closest I’ve come to that feeling is on hallucinogenic drugs in highschool. The feeling that everything’s cool, but nothing’s cool at the same time. A profound physical sense of emotional connection to the universe and loss of that connection at the same time.
That’s the first nightmare I can remember … except I’d call it more of a daymare since it could happen at any time when I closed my eyes (luckily less and less frequently as the years pass).
That’s the first time I’ve ever talked (or written) about that.
If her memory was anything like mine, she didn’t need to know what chickens were. She just needed to record the visual stimuli . . . and years later, when she learned about chickens, she was able to put a verbal label on the experience. You don’t need to know that verbal label in order to remember.
Like my experience. I obviously didn’t know what a space cube was, but then when I learned about space cubes I was able to confidently call them space cubes!
Yeah, I know, I’ll shut up about my space cubes now.
Look up something called ‘micropsia’ or ‘macropsia’. It’s a neurological condition that makes people feel very big or very tiny, and can be caused other conditions such as migraines. (also, check ‘Alice in Wonderland’ syndrome).