Antenna Radiation pattern multi-lobed and directional with increase in ground plane!

Why does an antenna radiation pattern become multi-lobed and directional, when the length of the ground plane is increased?
1. Does it increase the electrical length of the antenna?
2. Does an increase in ground plane lead to more reflections, leading to a more directional pattern?
3. How can one increase an antenna's Directivity in the desired direction?

Some effects might be specific to your design. Show a picture what antenna and ground plane size you refer to.

Thank you Volker for replying!

My antenna is designed for laptop applications. It is a folded PIFA.

The image attached shows you a sketch of what it is like.

For some antenna types the ground behave as a reflector, and if the ground is bigger, the antenna gets higher gain in the opposite direction.
There is a limit of the ground dimensions (usually about 3λ) from where the ground have no influence if is bigger.
Also (for some antenna types) the distance between antenna and ground can affect the side lobes. Adjusting this distance can search for a minimum amplitude of the side lobes.

The PIFA itself has rather omnidirectional pattern. With the ground plane, you get something like a tilted donut pattern, with a deep zero diagonal across the ground. At least that's what I have seen when simulating such designs. I would say the ground plane shorts the E field in that direction across the board.