Crullers are fried pâte à choux dough. When baked, rather than fried, this same dough can be used to make éclairs and cream puffs. A generous poof, in either form of cooking, comes from having the right balance between dough consistency and steam formation.
Pâte à choux creations are somewhat unique in that they are cooked twice - once during the mixing of the dough - once in the frying or the baking.
Recipes for the dough vary a little bit, but basically rely on using equal parts water and eggs with half as much butter and flour (by weight). Some recipes use half water and half milk (my preference). OK, sure there might be other ingredients in minor roles - salt, sugar, spice - but the basic ingredients are liquid, butter, flour, egg.
It's difficult to diagnose a problem without knowing your recipe or technique but I'll try to give a few pointers. After you bring the water and butter to a simmer (easy with the heat), add your flour and stir constantly - the way to tell when this cooking is done is not by time, but by observation - remove your paste from the heat when you can see the paste pulling away from the sides of the pot. There is a point of equilibrium in the process where sufficient water has been absorbed by the flour and a little water has been driven-off as steam - when this equilibrium is reached, the paste in the pot will visibly pull away from the pan as if there were some strange repellent force there.
Next, some recipes call for you to transfer the hot paste to a stand mixer - I have always mixed the eggs in by hand (mixing in by hand give me a better feel for the consistency of the finished dough). Add-in the eggs one-at-a-time and with constant stirring to fully incorporate in stages. My own preference here is to combine all the eggs in a separate bowl and beat them together first then pour the egg slowly into the paste while stirring vigorously.
Now, as far as dense crullers are concerned, my guess would be that one of three things happened - either you were heavy-handed with the flour or your failed to cook the water/butter/flour paste long enough or you didn't add enough egg to get a smooth, easy-to-pipe dough. Your dough should not drip off your spoon, but it should be fluid enough to pipe very easily when you use the piping bag to extrude your crullers. If your dough is not sufficiently fluid, it cannot expand from the steam inside to create the desirable poof.
My recommendations: Use bread flour if you didn't before (AP flour will work, I prefer bread flour), consider replacing half of your liquid with milk (totally optional - just offering another personal prejudice here), and consider weighing-out your flour if you used a volumetric measure before. Keep in mind that you need to shoot for equal parts liquid and egg, with half that amount of butter and flour (by weight) - for example 8 oz. water (1 C), 4 oz. butter (8 Tbsp.), 4 oz. flour (if your recipe calls for a cup, my cups of flour always weigh more than 4 oz.), and 4 or 5 eggs (about 8 oz. worth).
Finally - keep adding egg until the dough consistency is right even if you have to add more egg than the recipe calls for. If you don't have a scale to weigh-out your flour, go ahead with your volumetric measure but be aware that you might need to add some extra egg to your dough to get a smooth and fluid consistency - and if you add extra egg here, remember that a whole extra egg might be too much - beat a whole egg before adding it to your dough so you can incorporate it in stages to get you a consistency that you can easily pipe-out.
