How undersized do you want to get?
The Mightly Flea is just too darn small. It's hard to play, but it's fun. My biggest issue is throwing it, but once I throw it, I can do some basics with it, but man, if i hit the string wrong once... well, time to snap start it and try again. 25mm in diameter, smaller than a US Quarter, dense and heavy for it's size.
The Aoda Littles mention here might get this post yanked, but oh well. It's small, it's fun, it plays like a big yoyo, yet is clearly a small yoyo. It's definitely pocket sized, pocket-friendly and uses full sized strings. The small bearing and gap won't really allow for massive string layers though because of the full sized string. It needs Type 6 string for sure, type 8 is too big. THinner string might be ideal. It can be siliconed. At 35mm, it's still very small. Stepping up in size is the 50mm Magic Pear Ball, also known as the YYF JK, which I really like. I don't have the JK though.
If you're into YYJ's, of what I have, the Hitman Pro and XCon pro are in that size category, and of the 2, I like the Hitman II better, although the XCon Pro is a great trainer yoyo that forces you to improve your throw.
Of YYF's I have, the dv888 is all I got. Nothing amazing, but very decent and a well rounded player.
The Crucial Half & Half is nice but really shines using a Crucual Grooved Bearing rather than the flat it comes with. However, no longer in production, BST is the only way to get this Delrin yoyo.
One Drop has the MMN, Cafe Racer and the Dietz in their undersized category. I have a Cafe Racer coming my way, but I do nave the MMN and Dietz and those are both amazing. It is a preference thing as the MMN and Dietz play similar but have differences.
CLYW has the Campfire at 48mm and the Wooly Marmot at 50.2mm. Amazing stuff, no doubt. I prefer the slightly larger size and weight distribution of the Wooly Marmot.
So, that's my run-down of what I have in the undersized category. I prefer full sized/bordering over-sized yoyos that tend to be 67 grams or heavier. Undersized is fun for me to mix it up.