Benjxn mentioned maybe the diameter is larger. That would definitely have an effect. A larger diameter means larger circumference which means slower time due to altered taller gearing. Conversely, a smaller diameter with a smaller circumference would lower the gearing and improve acceleration at the expense of increased odometer readings. Just because you have a plus 1 or plus 2 tire/wheel combo does not mean there is no difference in overall diameter.

Again on the weight thing, rotational inertia increases with the square of the radius or something like that. The fact that the wide heaviest part of the rim (where the tire is mounted) is farther from the axle than a 16" wheel is what causes slower times.

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