Great question — answering as the UphillTech team:
Based on my personal experience with GP1 (I need to buy 2XL or 3XL gloves because of the length, I have the L on my fun bike, my wife the S), the length of the hand matters more. You have the freedom to move your hands inwards but the small one give the impression to hold a narrow stick.
Full answer by Rеnаud: https://bicycles.stackexchange.com/a/97074 (CC BY-SA 4.0).
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