If your kettle was the size of a single cup of water. It would boil in a few seconds. Instead it has a lot of empty space that takes longer to heat up, and raise the temperature to boiling. Adding more water just fills that empty space with more water to boil, lengthening the process.
The Keurig uses a small reservoir, separate from the main water tank, that is more efficient due to being much smaller, and therefore faster to heat. Essentially a really tiny single-cup kettle, or drip straw, inside the machine, refilled from the large tank you see on the side.
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