Pulse cranberries in a food processor for 10 short bursts until coarsely chopped (about 30 seconds).
Transfer to a large bowl, stir in sugar until coated, then cover and refrigerate 30 minutes, stirring once for even sweetness.
Drain crushed pineapple thoroughly, add to cranberry mixture, discarding juice.
Fold in miniature marshmallows and pecans, then gently incorporate the whipped topping until blend is uniform.
Cover and chill at least 1 hour (up to 4 hours for silkier texture) before serving.
Notes
Store in an airtight container in the refrigerator for up to 3 days. Keep below 40°F to maintain freshness and prevent bacterial growth.For best texture, consume within 48 hours—marshmallows begin to soften after day two.