From my perspective: I have huge hands... Certain guns (very few) are pretty wide (some Sigs come to mind) for even my hands and thinner grips give that extra measure of control and grip. Try a Sig P226 with wood grips - it feels like a plank. Thinner grips make it a lot more ergonomic, especially someone with smaller hands than I. Palm swells have the same purpose (ergonomics and control) and can be thinner or OEM grip size (75B palm swell grips, for example - can give you a little more grip with the familiarity of the OEMs.
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