Panels of brochures are never the same size - some are smaller than others so that the brochure can fold and close as evenly as possible. Generally, the difference is only 1 mm - 3 mm between panels.
Before starting the design process of your brochure, ensure that you (or your designer) are using the correct template for the size and fold type you will be ordering. As brochure templates are not interchangeable, it is important to use the right one before creating or placing any content so that you do not have to rearrange your design later on. For example, you cannot design a brochure using a 3-panel Trifold template and then place an order for 3-panel Z Fold brochures. Your design will need to be adjusted to match the folding lines / correct panel sizes.