Are you using volume measurements (cups) or weight?
If you are using volume, just multiplying the "cups" used can cause too much ingredients to be used, for example, if your method of measuring adds a 1/2 ounce to each cup (not uncommon), then when multiplying the recipe, it can add up to way too much of that ingredient.
Also the amount of leavening needs to change according to the size of your pan. For example, you'll see just a little difference going from an 8" to 9" to 10" pan, but if you double a recipe and start using much larger pans, the recipe will not perform the same.
The Cake Bible by Rose Levy Beranbaum explains this better than me.