Are there any undersized plastic throws BESIDES the Mosquito or Dragonfly?
Capella,Ceephax ect. There really aren’t that many out there since plastic isn’t very dense its lightweight so its hard to achieve a good weight distribution while maintaining the right weight.
UHF one. Can’t think of anything good.
The dragonfly is regular sized. The Flying Squirrel is a Mosquito with weight rings, making it much better.
Full plastic? Hard to get the right kind of weight for a full plastic.
Something considered a full plastic(buried metal weight rings hidden in the yoyo)? Not sure of any in that size category.
Metal/Plastic? With the aluminum or metal components, it’s better. XCon Pro and Hitman Pro come to mind immediately. The XCon is a great “make you improve your game” yoyo as well as a tremendous performer. The Hitman Pro has some issues with the rims coming off or loose, so they may need to be pressed back on, but other than that, no issues I am aware of.
The Vigilante is a hybrid, but it is a 50mm throw. I’ve heard some people hate on it, but I really enjoy playing with mine. It may be a LITTLE overpriced, because it feels sort of cheap, but it is a great performer nonetheless. 
ONE
The only ones I have seen were reshaped.
The Crucial Half and Half is an amazingly good undersized plastic. Probably one of my favorite throws.
The Flipside is sorta small, The PSG and the Awesome (when beefcaked) Flying Squirrel
Menehune, Mini Mo/tu if you count those…
I believe crucial’s cream, milk, and milk 2 were all undersized.
the YYJ lyn fury comes to mind as well.
X-con Pro, if you count Bi-Materials. It plays better than some of my full-size throws.
I think those Crucial’s are in the full sized league.
I stand (partially) corrected. The Milk and Milk 2% were 56mm in diameter, where are the cream was 52mm in diameter.
Here too…