The difference comes up front(triple vs. double). His lowest gear is probably something like 34 to 26, where you might have 26 to 26. So, his lowest gear is about 50% higher than yours.
The advantage of a 'taller' cassette, like 11-34, is to have a broader range of gears. The advantage of a tighter cassette is the ability to make small jumps between gears, and dial in the perfect cadence.
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