I think the main mechanism is that it doesn't get absorbed. For example, sugary urine is a well-understood side effect of diabetes that isn't well-managed, caused by your kidney draining excess sugar from your bloodstream into your urine.

Interestingly enough, your body also raises its level of activity in order to burn calories and maintain weight levels. I believe they've measured a surprising amount of excess calories burned from extra fidgeting when overfeeding people relative to their usual diet.