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“Is my watch too big?” is a constant question on forums dedicated to smartwatches and fitness trackers of all kinds. (Some, but fewer, wonder if their watch is too small.) Personally, I don’t much care how a watch looks , but I’ve noticed that size can make a big difference in how accurately a watch functions. If your watch is too big, your numbers may be off.

How smartwatch size affects heart rate accuracy

Garmin Forerunner 265 (larger) in red, 265s (smaller) in pink. Chest strap for reference in black. Credit: Beth Skwarecki

Watches measure heart rate through an optical sensor on the back of the watch. (That’s what you see glowing green back there.) The heart rate sensor detects how that light reflects off the blood vessels in your skin.…