Most of the quick weight "re-gain" that people experience when they fall off the wagon or "cheat" is bloat/water weight. Carbs will make you hold onto water, and if you've been in a cycle of being fairly restrictive with your carbs, binging or splurging on them will often cause a water weight gain.

For many people it's true that over time--unless you're willing to up your exercise and/or lower your calories--the higher carbs will probably make you gain actual "fat" weight. But the quick initial gains are usually just water.