In my humble opinion, conceptually there are three tiers of “undersized” based on diameter:
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REALLY tiny, made entirely of very heavy metals like stainless steel or brass. 25mm-30mm diameter or less. This is getting into serious novelty throw territory. Examples: Heist, Flea, Aoda Little.
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Undersize, 31mm-49mm diameter – to me, this is “mainstream” undersized today. Tons of examples, C3 Token, TopYo Photon, Mini-Star 2.
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Moderately undersize, basically a 2010 idea of diameter and width, more of a “throwback” design – as yo-yos clearly have been getting bigger over time, both in diameter and width. 50mm-53mm diameter. Examples are Veritas Pro, Hexada, Anti-Yo, 888, Marquis.
Here’s a picture of representative family members in each tier:
I’d say the typical / average diameter of a yo-yo today is “Shutter diameter” or 56mm, but sometimes even more. The Horizon is 59mm.