Preheat the oven to 350°F (175°C). Line a muffin tin with muffin liners or grease with cooking spray.
In a large mixing bowl, whisk together 1 ½ cups of flour, baking powder, baking soda, salt, 1 teaspoon cinnamon, and nutmeg.
In another bowl, combine the granulated sugar, brown sugar, pumpkin puree, vegetable oil, eggs, and vanilla extract. Mix until well combined.
Gradually add the wet ingredients to the dry ingredients, stirring gently until just combined. Be careful not to overmix; lumps are okay.
In a small bowl, prepare the streusel topping by mixing ½ cup chopped pecans, ¼ cup flour, ¼ cup brown sugar, melted butter, and ½ teaspoon cinnamon until crumbly.
Divide the muffin batter evenly into the prepared muffin tin, filling each cup about two-thirds full. Sprinkle the streusel mixture generously on top of each muffin.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.