A long surfboard is generally easier to ride as they offer more stability and volume, making them a great option for beginners. This allows a surfer who is just starting out to get comfortable with standing up and learning the basics.
As you progress, transitioning to a mini mal is great for that same stability with the additional flexibility to ride a variety of wave sizes. With that being said, shorter boards tend to be harder to control which is why they are a better fit for more advanced surfers.