That’s a good question! The potato is about 80% water and about 20% solids or starch. The starch in the potato will convert to sugar if stored at too cold of a temperature. If the potato is properly stored (45-48 degrees F is ideal), neither will accumulate sugar. Mashed potatoes are usually combined with a liquid, which dilutes the potato and thus the sugar when comparing equal quantities.

Here is a link to nutritional information for a baked potato. You can do the same for mashed potatoes.