Planar Inverted F Antenna Sizes for Mobile Communications
Ground plane size is one of the many design parameters, and affects efficency and bandwidth. So, you can't pick it freely for reasons of wanting the PIFA to act as good antenna at all the frequencies you need.

Also, size of any antenna part in a mobile device is usually subject to very substantial mechanical limitations; so you literally can't have the perfect ground plane, simply because you're constrained in size.

PIFAs are about \$\frac\lambda4\$ in extension, there's little you can do about that, unless you don't need efficiency (but you need efficient antennas in mobile devices), or can work with capacitive loading because you don't need much bandwidth; in that case, you'd not get better than \$\frac\lambda8\$.

That wavelength is measured mostly (not completely) w.r.t. to the dielectric between PIFA and ground plane. In any mobile device I can think of (mostly: phones), there's no special high-\$\epsilon\$ materials embedded between antenna and board, so that this wavelengths is the free space wave length.

So, probably, the fact that the ground plane is larger than the PIFA part is simply because it can be larger, whereas the PIFA is mechanically limited. 

Other than that, mobile device antenans are not "realized on paper and produced as manually analyzed" but very much iteratively designed, simulated, improved. So, really, antennas in mobile devices, especially in mobile phones, can be the result of very many tradeoffs. For example: Someone designed it for usage while held against an ear. That worked well, but when they simulated how it'd work if there was no fleshy head nearby (as in hands-free operation of phones), it worked really bad. So the designer chose to do whatever it took to make it work slightly better in that case, maybe at the expense of working not as great in the ear-held case. 

So, really, looking at a real-world integrated antenna design and asking "what's the singular reason for this shape" won't work. There's going to be many reasons, for many tradeoffs you simply can't see in the finished product.