Spark's calculator takes into account your body weight, on the assumption that heavier people burn more. This holds true for many (most?) forms of exercise, including walking, aerobics, etc.
However, swimming is not a weight bearing exercise (you naturally float in water), and this can introduce a distortion into Spark's calculations, especially for heavier people.
Also, as a reality check, most people burn between 6 and 10 calories a minute (360-600 an hour) through sustatined exercise. 6 at the more moderate end, 10 being more vigorous. Someone who is pretty fit and really pushing things might get to 12-13 a minute. Figures above 800 an hour should be regarded with a great deal of suspicion. 1450 an hour can safely be assumed to be the result of some faulty assumptions, and is NOT a realistic figure.