It's possible your baking soda has expired! Check the expiration date. Once opened, baking soda is good for about 6 months. If you're sure your baking soda is fine, it's likely an issue of pan size. This banana bread baked in an 8x4" loaf pan will seem taller than a banana bread baked in a 9x5" pan. Both will be delicious, but one will seem taller than the other.